Singapore's Joseph Schooling made life harder for Michael Phelps after beating the master in their 100 meters butterfly heats on Thursday and going through to the semi-finals with the fastest time.
The Texas-based swimmer, a 100m butterfly bronze medalist at last year's world championships, set a time of 51.41 seconds with boyhood idol Phelps second in the heat with a 51.60 that left him fourth overall.
"It feels like a morning swim. Relaxed, easy," Schooling told reporters.
Phelps has already amassed three golds at the Rio Games and has other priorities, balancing the need to qualify comfortably without damaging his chances for the evening men's 200m individual medley final.
If the American wins that event, for his 22nd Olympic gold and 26th medal, he will become the first swimmer to win the same event at four successive Games and only the third athlete after discus thrower Al Oerter and long jumper Carl Lewis.
If that fails, the 100m butterfly gives him a second chance to pull off the feat as he has also won that at the last three Games and holds the world record of 49.82.
A medal is the main aim for the day but qualifying in the second semi-final heat would have made his life easier with the IM scheduled for 11 p.m. local and the butterfly semi-finals starting at 11.34.
Phelps admitted he had messed up, even if the time was acceptable, because he now faced being in the opening semi. An odd finish number would have put him in the later group.
"I like to have those extra seven or eight minutes in between," said the American. "It's another 300 (meters) I could do in the warm-down pool.
Michael Phelps Olympic history
Michael Phelps Olympic history
Team USA swimmer Michael Phelps prepares to swim laps during a practice session at Northside Swim Center in San Antonio, Texas, July 16, 2016. Mandatory Credit: Soobum Im-USA TODAY Sports/File Photo
Michael Phelps (C) of the U.S. swims the butterfly stroke during his team's victory in the men's 4x100 meters medley relay at the National Aquatics Center during the Beijing 2008 Olympics August 17, 2008. Picture taken with underwater camera. REUTERS/Wolfgang Rattay/File photo
Michael Phelps of the U.S. poses with his gold medal after winning the men's 4x100m medley relay final during the London 2012 Olympic Games at the Aquatics Centre August 4, 2012. REUTERS/Jorge Silva/File photo
Michael Phelps and Garrett Weber-Gale (L) of the U.S. celebrate after winning the men's 4x100m freestyle relay swimming final at the National Aquatics Center during the Beijing 2008 Olympic Games August 11, 2008. REUTERS/David Gray (CHINA)
Gold medallist Michael Phelps of the U.S. listens to his national anthem during the men's 200m individual medley victory ceremony during the London 2012 Olympic Games at the Aquatics Centre August 2, 2012. REUTERS/David Gray
Members of the men's 4 x 200m freestyle relay team, Michael Phelps, Conor Dwyer, Ryan Lochte and Ricky Berens (L-R) hold their national flag as they celebrate their gold medal win in the men's 4x200m freestyle relay victory ceremony during the London 2012 Olympic Games at the Aquatics Centre July 31, 2012. Phelps won the record 19th Olympic medal on Tuesday when he joined forces with his U.S. team mates to win the 4x200m freestyle relay. REUTERS/Jorge Silva
Michael Phelps of the U.S. competes in the men's 400m individual medley swimming final at the National Aquatics Center during the Beijing 2008 Olympic Games August 10, 2008. Picture taken with underwater camera. REUTERS/Wolfgang Rattay (CHINA)
Michael Phelps of the U.S. holds up his award recognising him as the most decorated Olympian, during the London 2012 Olympic Games at the Aquatics Centre August 4, 2012. Phelps ended his incredible Olympic career on the perfect note on Saturday, winning his 18th gold medal for the United States in the men's medley relay, the last time he will swim a competitive race. REUTERS/Brian Snyder
A combination photo shows Michael Phelps of the U.S. holding each of his eight gold medals in the swimming competition at the Beijing 2008 Olympic Games. Phelps won his eighth gold medal on Sunday, beating Mark Spitz's record of seven gold medals won at a single Games in 1972. The medals were won in: (Top row L-R) 400m individual medley (August 10), 4x100m freestyle relay (August 11), 200m freestyle (August 12), 200m butterfly (August 13). (Bottom row L-R) 4x200m freestyle relay (August 13), 200m individual medley (August 15), 100m butterfly (August 16), and 4x100m medley relay (August 17). REUTERS/Staff (CHINA)
Michael Phelps of the U.S. looks down at the pool after his team was beaten to the gold by France in the men's 4x100m freestyle relay final during the London 2012 Olympic Games at the Aquatics Centre July 29, 2012. REUTERS/David Gray
Photographers surround Michael Phelps of the U.S. after he won his eighth gold medal of the games at the National Aquatics Center during the Beijing 2008 Olympic Games, August 17, 2008. REUTERS/Gary Hershorn (CHINA)
Discover More Like This
BACK TO SLIDE
"Obviously in the first swim I needed to take care of business and then whatever I can do in the second one just to make it through is what I'll do tonight."
Hungary's Laszlo Cseh, who boasts the fastest time of the year so far, was second fastest with Tom Shields of the United States third.
South Africa's world champion Chad Le Clos, who shared the 100m butterfly silver medal in London with Russia's Yevgeny Korotyshkin, was seventh.
Singapore had two swimmers in the semis with Quah Zheng Wen qualifying 16th with a personal best of 52.08.