When asked why it needs to be qualified, McEnroe said, "Well because if she was in, if she played the men's circuit she'd be like 700 in the world."
McEnroe continued, suggesting Williams could beat some men's players because of her mental fortitude. But he said he doesn't think she's athletic enough to beat men at a high rate.
"That doesn't mean I don't think Serena is an incredible player. I do, but the reality of what would happen would be I think something that perhaps it'd be a little higher, perhaps it'd be a little lower. And on a given day, Serena could beat some players. I believe because she's so incredibly strong mentally that she could overcome some situations where players would choke 'cause she's been in it so many times, so many situations at Wimbledon, The U.S. Open, etc. But if she had to just play the circuit — the men's circuit — that would be an entirely different story.
"Maybe at some point a women's tennis player can be better than anybody. I just haven't seen it in any other sport, and I haven't seen it in tennis. I suppose anything's possible at some stage."
RELATED: Serena and Venus Williams through the years
Venus and Serena Williams through the years
Venus and Serena Williams through the years
Venus Williams (L) and her sister Serena of the U.S. celebrate their victory in the doubles finals against Mariaan de Swardt of South Africa and Elena Tatarkova of the Ukraine at the European indoor championships in Kloten October 18. The Williams sisters won 5-7 6-1 6-3.
Serena Williams (back) goes after a second set shot from her sister Venus Williams March 28 in the final match at the Lipton Championships. Venus defeated Serena 6-1 4-6 6-4 to take the title.
Serena Williams from the United States (R) holds the Grand Slam Cup trophy after the final match against her sister Venus at the Grand Slam Cup in Munich, October 3. Serena won the match 6-1 3-6 6-4.
USA'S Serena Williams (L) and sister Venus practice for the Olympic Games in Sydney, September 21, 2000. The Williams sisters are competing for the United States in the Games of the XXVII Olympiad.
Venus Williams (R) embraces her sister Serena Williams after she won the women's final at the U.S. Open Tennis Championship in New York September 8, 2001. Venus Williams won 6-2 6-4 in a sixty-nine minute match, repeating her final's win from last year. Tonight's final is the first Grand Slam final contested by sisters since Maud Watson beat Lilian Watson 117 years ago in the first Wimbledon women's final in 1884. REUTERS/Gary Hershorn
Venus (R) and Serena Williams of the U.S. confer during their women's doubles match [against Slovenia's Tina Krizan and Katarina Srebotnik] at the Wimbledon tennis championships, July 5, 2002. The Williams sisters won 6-2 6-0.
Serena Williams of the United States returns to her sister [Venus] during the women's final at the U.S. Open in Flushing Meadows, New York, September 7, 2002.
Serena Williams of the United States (L) is greeted at the net by her
sister Venus following the women's final at the U.S. Open in Flushing,
New York, September 7, 2002. Serena won the match 6-4 6-3 to capture
the U.S. Open title. REUTERS/Ray Stubblebine
Venus (R) and Serena Williams (L) of the U.S. play third round women's
doubles against Russia's Elena Dementieva and Lina Krasnoroutskaya at
the Wimbledon Tennis Championships in London, July 1, 2003.
U.S tennis players Serena (L) and Venus Williams smile during a children's tennis practice session on court at Wimbledon in south west london, June 17, 2004. The Wimbledon championships begin on June 21. REUTERS/Kieran Doherty KD/AA
Serena Williams reacts after a missed point against sister [Venus] in their fourth round match at the U.S. Open tennis tournament in Flushing Meadows, New York, September 4, 2005. [Venus] defeated Serena 7-6 6-2.
Venus Williams of the U.S hits a return to sister Serena Williams during their fourth round match at the U.S. Open tennis tournament in Flushing Meadows, New York, September 4, 2005. REUTERS/Mike Segar JA/mk
Serena (L) and Venus Williams of the U.S. reach for the ball during their semi-finals doubles match against Nathalie Dechy of France and Casey Dellacqua of Australia at the Wimbledon tennis championships in London July 4, 2008. REUTERS/Toby Melville (BRITAIN)
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 August 17, 2008. REUTERS/Toby Melville (CHINA)
Serena Williams (L) of the U.S. and her sister Venus talk, after winning their women's doubles final match against Slovakia's Daniela Hantuchova and Japan's Ai Sugiyama, at the Australian Open tennis tournament in Melbourne January 30, 2009. REUTERS/Petar Kujundzic (AUSTRALIA)
Serena Williams of the U.S. serves the ball to sister and compatriot Venus Williams during their semi-final match at the WTA Dubai Tennis Championships February 20, 2009. REUTERS/Ahmed Jadallah (UNITED ARAB EMIRATES)
Venus Williams of the U.S. serves the ball to sister and compatriot Serena Williams during their semi-final match at the WTA Dubai Tennis Championships February 20, 2009. REUTERS/Ahmed Jadallah (UNITED ARAB EMIRATES)
Venus Williams of the U.S. (L) and Serena Williams of the U.S. pose for a photograph before their Ladies' Singles finals match at the Wimbledon tennis championships in London, July 4, 2009. REUTERS/Eddie Keogh (BRITAIN SPORT TENNIS)
Serena Williams (L) of the U.S. grimaces after being hit by a serve from her sister Venus during their doubles match against Julia Goerges of Germany and Arantxa Parra Santonja of Spain at the U.S. Open tennis championships in New York September 3, 2009. REUTERS/Kevin Lamarque (UNITED STATES SPORT TENNIS)
Sisters Serena Williams (R) and Venus Williams of the U.S. celebrate after defeating Czech Republic's Andrea Hlavackova and Lucie Hradecka in the women's doubles tennis gold medal match at the All England Lawn Tennis Club during the London 2012 Olympic Games August 5, 2012. REUTERS/Stefan Wermuth (BRITAIN - Tags: OLYMPICS SPORT TENNIS TPX IMAGES OF THE DAY)
U.S tennis players Serena Williams (L) and Venus Williams look on during a news conference in Nigeria's commercial capital Lagos October 31, 2012. REUTERS/Akintunde Akinleye (NIGERIA - Tags: SPORT TENNIS)
Venus Williams (top) serves as she and her sister Serena of the U.S. play doubles against compatriots Raquel Kops-Jones and Abigail Spears at the U.S. Open tennis championships in New York August 31, 2013. REUTERS/Ray Stubblebine (UNITED STATES - Tags: SPORT TENNIS)
Tennis - Wimbledon - All England Lawn Tennis & Croquet Club, Wimbledon, England - 6/7/15
Women's Singles - USA's Serena Williams and USA's Venus Williams embrace after their fourth round match
Action Images via Reuters / Andrew Couldridge
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Serena Williams of the U.S. follows the flight of the ball as she falls on a return shot to her sister and compatriot Venus Williams during their quarterfinals match at the U.S. Open Championships tennis tournament in New York, September 8, 2015. REUTERS/Carlo Allegri
Britain Tennis - Wimbledon - All England Lawn Tennis & Croquet Club, Wimbledon, England - 9/7/16 USA's Serena Williams and Venus Williams celebrate winning their womens doubles final against Hungary's Timea Babos and Kazakhstan's Yaroslava Shvedova with the trophies REUTERS/Tony O'Brien
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McEnroe also pointed to the time differences in men's and women's track races as an example of the physical gap.
According to ESPN, McEnroe called Williams "arguably the greatest athlete of the last 100 years" after she won Wimbledon in 2015.
Asked about whether he could beat Williams, McEnroe joked that now that she's pregnant, it might be his chance.