Serena Williams reaches up to hit the ball during the Nancy Reagan Tennis tournament.

(Ken Levine via Getty Images)

Tracy Austin (center) stands with sisters Venus Williams and Serena Williams at the Nancy Reagan Tennis tournament.

(Ken Levine via Getty Images)

Sisters Serena, left, and Venus Williams shake hands after a game 1991 in Compton, CA. Serena and Venus Williams will be playing against each other for the first time July 6, 2000 in the tennis semifinals at Wimbledon.

(Photo by Paul Harris/Online USA)

Serena Williams in action on the tennis court.

(Photo By Ken Levine/Getty Images)

Serena Williams and her sister Venus Williams ride with their father Richard Williams at a tennis camp in Florida.

(Ken Levine /Allsport)

Sisters, Venus (L) and Serena Williams, share the same racquet as they play in the Arthur Ashe Kids Day tennis exhibition at the USTA National Tennis Center in New York August 29. Serena is competing in her first U.S. Open when competition begins August 31. Venus made it to the finals in the 1997 U.S. Open.

(Jeff Christensen / Reuters)

Serena Williams from USA at the 1998 Roland Garros French Open.

(Photo by Franck Seguin/Corbis/VCG via Getty Images)

Serena Williams of the USA relaxes against a palm tree during a break from the Lipton Championships in Key Biscayne, Florida, USA.

(Clive Brunskill/Allsport)

Brian Littrell, AJ McLean, Howie Dorough, Serena Williams, Venus Williams, Kevin Richardson, and Nick Carter.

(Photo by Ron Galella/WireImage)

Candid pictures of Serena Williams during the Chase Championships in New York City, New York. 

(Clive Brunskill /Allsport)

Serena Williams of the United States in action during the Adidas International Tournament played at White City in Sydney, Australia.

(Clive Brunskill /Allsport)

American tennis player Serena Williams (left) and her sister Venus (right) with their mother Oracene, Key Biscayne, Florida, 28th March 1999. The sisters have just played each other in the final of the Lipton International Players Championships (now the Sony Ericsson Open), with Venus defeating Serena to win the tournament. This was the first time that two sisters had competed against each other in a final.

(Photo by Georges De Keerle/Getty Images)

NEW YORK, UNITED STATES - US OPEN 1999

Serena WILLIAMS

(Photo by Mark Sandten/Bongarts/Getty Images)

Sports Illustratated Sportsman of the Year 2000 award at the Beacon Theater in New York, United States on December 14, 2000 - Venus and Serena Williams.

(Photo by David LEFRANC/Gamma-Rapho via Getty Images)

Tennis star Serena Williams arrives at The Theater at Madison Square Garden for the Essence Awards 2001.

(Photo by Richard Corkery/NY Daily News Archive via Getty Images)

Serena Williams of the USA shows a smile in her match with Jennifer Capriati of the USA in the womens singles during the semi finals of the French Open at Roland Garros in Paris on June 6, 2002.

(Photo by Phil Cole/Getty Images)

Serena Williams and Michael J. Fox during People Magazine Heroes Among Us-A celebration of the Human Spirit Luncheonrit at Heroes Among Us in New York, New York, United States.

(Photo by Mychal Watts/WireImage)

US Serena Williams, who won the Roland-Garros Tennis Open in 2002, gives a press conference, 25 May 2003 in Paris, one day ahead of the beginning of the French Tennis Open.

(JEAN-LOUP GAUTREAU/AFP/Getty Images)

Serena Williams of the USA and Lleyton Hewitt of Australia with their 2002 world number one awards during the ITF World Champions dinner on June 2, 2003 at the Pavillon d'Armenonville in Paris, France.

(Photo by Clive Brunskill/Getty Images)

Serena Williams during 34th NAACP Image Awards - Press Room at Universal Amphitheatre in Universal City, California, United States.

(Photo by Ron Galella/WireImage)

Serena Williams of the USA announces her withdrawal from the State Farm Women's Tennis Classic because of a knee injury February 27, 2003, in Scottsdale, Arizona. 

(Photo by Brian Bahr/Getty Images)

Serena Williams during 'Ocean's Twelve' Los Angeles Premiere - Arrivals at Grauman's Chineese Theater in Los Angeles, California, United States.

(Photo by Jon Kopaloff/FilmMagic)

Serena Williams signs autographs after playing her sister Venus on the Williams Sisters Tour on December 2, 2004 at Philips Arena in Atlanta, Georgia. 

(Photo by Scott Cunningham/ Getty Images)

Serena Williams and Usher during VH1 Big in '04 - Backstage and Audience at Shrine Auditorium in Los Angeles, California, United States.

(Photo by Jeff Kravitz/FilmMagic, Inc)

Serena Williams returns a shot during her fourth-round 2006 U.S. Open match against Amelie Mauresmo at Arthur Ashe Stadium in Flushing Meadows-Corona Park. Williams lost, 6-4, 0-6, 6-2.

(Photo by Corey Sipkin/NY Daily News Archive via Getty Images)

Serena Williams during Jed Weinstein Presents and Epic Sports Celebrate the New Face of Land Rover with Maria Sharapova and Serena Williams at Manhattan Automobile Company Rooftop in New York City, New York, United States.

(Photo by Ryan Born/WireImage)

Serena Williams of USA attends a signing session held at the HP store on November 12, 2008 in Sandton, Johannesburg, South Africa.

(Photo by Lefty Shivambu/Gallo Images/Getty Images)

Gold medallists Serena (R) and Venus Williams of the U.S. celebrate on the podium after the women's doubles tennis competition at the Beijing 2008 Olympic Games.

(REUTERS/Toby Melville)

Serena Williams of the U.S. poses for photographers as she holds the winners trophy after defeating Russia's Vera Zvonareva in the womens' singles final at the 2010 Wimbledon tennis championships in London, July 3, 2010.

(REUTERS/Stefan Wermuth)

Tennis star Serena Williams watches a game between the Miami Heat and the Charlotte Bobcats at American Airlines Arena on November 19, 2010 in Miami, Florida.

(Photo by Mike Ehrmann/Getty Images)

Serena Williams of United States talks to media during a visit to the Wheel of Brisbane in South Bank on day two of the Brisbane International on December 31, 2012 in Brisbane, Australia.

(Photo by Chris Hyde/Getty Images)

LATE NIGHT WITH JIMMY FALLON -- Episode 695

Pictured: Serena Williams during an interview with host Jimmy Fallon on September 10, 2012

(Photo by: Lloyd Bishop/NBC/NBCU Photo Bank via Getty Images)

Serena Williams of the United States looks on in her match against Varvara Lepchenko of the United States during day one of the Brisbane International at Pat Rafter Arena on December 30, 2012 in Brisbane, Australia.

(Photo by Matt Roberts/Getty Images)

Serena Williams of the United States poses for photographers with the Billy Jean King Trophy after defeating Maria Sharapova of Russia during the final of the TEB BNP Paribas WTA Championships at the Sinan Erdem Dome October 28, 2012 in Istanbul, Turkey.

(Photo by Matthew Stockman/Getty Images)

Serena Williams plays an exhibition match at the Buenos Aires Lawn Tennis Club on November 30, 2013 in Buenos Aires, Argentina.

(Photo by Lalo Yasky/WireImage)

Serena Williams of the US reacts during a WTA Championships final tennis match against China's Li Na on October 27, 2013 in Istanbul.

(BULENT KILIC/AFP/Getty Images)

Tennis players Serena Williams of the US (R) and Victoria Azarenka of Belarus (L) pose during a media opportunity in the Thai seaside resort town of Hua Hin on December 27, 2013 a day before the two women play in an exhibition tennis match.

(STR/AFP/Getty Images)

US tennis player Serena Williams poses with the Suzanne Lenglen trophy near the Eiffel Tower in Paris on June 8, 2013, after winning the Roland Garros French Tennis Open. Serena Williams won her second French Open title 11 years after her first triumph, defeating title-holder Maria Sharapova 6-4, 6-4 in a brief, but high-quality final. It was the 31-year-old American's 16th Grand Slam title win, taking her to within two of Chris Evert and Martina Navratilova who are tied for fourth on the all-time list.

(KENZO TRIBOUILLARD/AFP/Getty Images)

Serena Williams of the US holds the trophy as she celebrates after victory in her women's singles final match against Russia's Maria Sharapova on day thirteen of the 2015 Australian Open tennis tournament in Melbourne on January 31, 2015.

(MAL FAIRCLOUGH/AFP/Getty Images)

Serena Williams of the U.S. reacts during her women's singles match against Roberta Vinci of Italy at the French Open tennis tournament at the Roland Garros stadium in Paris June 2, 2013. World number one Serena Williams reached the quarter-finals of the French Open for the first time since 2010 with an easy 6-1 6-3 win over top-ranked doubles player Roberta Vinci on Sunday.

(REUTERS/Vincent Kessler)

Serena Williams of the United States holds the Daphne Akhurst Memorial Cup after winning the women's final match against Maria Sharapova of Russia during day 13 of the 2015 Australian Open at Melbourne Park on January 31, 2015 in Melbourne, Australia.

(Photo by Quinn Rooney/Getty Images)

Serena Williams of the United States and Maria Sharapova of Russia shake hands after Williams won their women's final during day 13 of the 2015 Australian Open at Melbourne Park on January 31, 2015 in Melbourne, Australia.

(Photo by Hannah Peters/Getty Images)

Serena Williams of the United States plays a forehand in the women's singles match against Agnieszka Radwanska of Poland during day seven of the 2015 Hopman Cup at Perth Arena on January 10, 2015 in Perth, Australia.

(Photo by Will Russell/Getty Images)

Serena Williams participates in The Serena Williams Ultimate Run on December 14, 2014 in Miami Beach, Florida.

(Photo by Johnny Louis/WireImage)

Serena Williams attends the First Annual Serena Williams Ultimate 5k Run on December 14, 2014 in Miami Beach, Florida.

(Photo by Manny Hernandez/Getty Images)

Serena Williams attends the Ultimate Run VIP Kick-Off on December 12, 2014 in Miami Beach, Florida.

(Photo by Aaron Davidson/WireImage)

Serena Williams is sighting on December 12, 2014 in Miami, Florida.

(Photo by Alo Ceballos/GC Images)

Serena Williams is seen on the Ultimate Run South Beach on December 14, 2014 in Miami, Florida.

(Photo by Alo Ceballos/GC Images)

Andre Agassi of the Singapore Slammers talks with team mates Daniela Hantuchova and Serena Williams during their teams match against the UAE Royals during the Coca-Cola International Premier Tennis League at the Singapore Sports Hub on December 4, 2014 in Singapore.

(Photo by Clive Brunskill/Getty Images for IPTL 2014)

Serena Williams of the Singapore Slammers in action against Ana Ivanovic of the Indian Aces during the Coca-Cola International Premier Tennis League at the Singapore Sports Hub on December 3, 2014 in Singapore.

(Photo by Clive Brunskill/Getty Images for IPTL 2014)

Serena Williams of the United States in action against Ana Ivanovic of Serbia in the Energi Danmark Champions Battle at Jyske Bank Boxen Arena on November 19, 2014 in Herning, Denmark.

(Photo by Lars Ronbog / FrontZoneSport via Getty Images)

Tennis players Caroline Wozniacki and Serena Williams attend a game between the New York Knicks and the Washington Wizards at Madison Square Garden on November 4, 2014 in New York City. The Wizards defeated the Knicks 98-83.

 (Photo by Jim McIsaac/Getty Images)

Serena Williams attends The 11th Annual CFDA/Vogue Fashion Fund Awards at Spring Studios on November 3, 2014 in New York City.

(Photo by Gary Gershoff/WireImage)

Serena Williams of USA celebrates with the Billie Jean King trophy after she defeats Simona Halep of Romania in the final during day seven of the BNP Paribas WTA Finals tennis at the Singapore Sports Hub on October 26, 2014 in Singapore.

(Photo by Julian Finney/Getty Images)

Serena Williams of the United States shows her frustrations during her straight sets defeat by Simona Halep of Romania in their round robin match during the BNP Paribas WTA Finals at Singapore Sports Hub on October 22, 2014 in Singapore.

(Photo by Clive Brunskill/Getty Images)

Katie Holmes, Francois Bennahmias and Serena Williams attend the Audemars Piguet Art Commission Presents 'Reconstruction of the Universe' By Sun Xun on November 29, 2016 in Miami Beach, Florida.

(Photo by Bryan Bedder/Getty Images for Audemars Piguet)

Venus Williams and Serena Williams speak to guests at the Healthy Compton 2016 Community Festival at MLK Transit Center Plaza on November 12, 2016 in Compton, California.

(Photo by Jerritt Clark/Getty Images)

Serena Williams is seen leaving the Versace show during Milan Fashion Week Spring/Summer 2017 on September 23, 2016 in Milan, Italy.

(Photo by Jacopo Raule/Getty Images)

Entertainer Beyonce and tennis player Serena Williams perform on stage during closing night of 'The Formation World Tour' at MetLife Stadium on October 7, 2016 in East Rutherford, New Jersey.

(Photo by Larry Busacca/PW/WireImage)

2016 U.S. Open - Day 11 Serena Williams of the United States serving against Karolina Pliskova of the Czech Republic in the Women's Singles Semifinal match on Arthur Ashe Stadium on day eleven of the 2016 US Open Tennis Tournament at the USTA Billie Jean King National Tennis Center on September 8, 2016 in Flushing, Queens, New York City.

(Photo by Tim Clayton/Corbis via Getty Images)