Simone Biles managed just one event before being pulled from the rest of the women's team final in the artistic gymnastics with what was described as a "medical issue".
The usually brilliant Biles struggled on the vault, landing awkwardly after making a hash of an Amanar and completing only a Yurchenko 1.5 twist.
Biles posted the lowest score of the first rotation on Tuesday, which saw Team USA fall behind the Russian Olympic Committee, with a 13.766.
In worrying scenes, Biles then left the floor with a trainer immediately after the first rotation, with the team in deep discussion. While she did return to the floor in a tracksuit, it was later announced that she would take no further part in proceedings.
"Simone Biles has withdrawn from the team final competition due to a medical issue," USA Gymnastics tweeted.
"She will be assessed daily to determine medical clearance for future competitions. Thinking of you, Simone!"
This event was the first of six for Biles at Tokyo 2020, where she entered as the overwhelming favourite to defend the all-around title she won at Rio 2016.
Uncertainty is now sure to surround Biles' fate for the rest of these Games with any further absence representing a huge blow, given her status as one of the marquee athletes for an Olympics delayed by a year due the coronavirus pandemic.
NBC reported during the broadcast that Biles' withdrawal was not related to injury and was down to a "mental issue she is having", according to comments from a Team USA coach.
The 24-year-old gymnast had revealed on Instagram how she felt the "weight of the world" was on her shoulders after helping Team USA qualify for the final in Tokyo.
"It wasn’t an easy day or my best but I got through it," Biles wrote.
"I truly do feel like I have the weight of the world on my shoulders at times. I know I brush it off and make it seem like pressure doesn’t affect me, but damn sometimes it's hard hahaha!
"The Olympics is no joke. BUT I'm happy my family was able to be with me virtually. They mean the world to me."
Biles: Olympic performance beyond my wildest dreams
Simone Biles feels she achieved beyond her "wildest dreams" at the Paris Olympic Games despite failing to add to her three gold medals for the year on Monday.
Having won gold in the team, all-around and vault events, Biles was looking to round off her trip to France with more success in Monday's balance beam and floor finals.
However, the Team USA star fell from the beam and finished fifth in the former event before missing out on gold in the latter, instead taking silver behind Brazil's Rebeca Andrade.
Overall, Biles has now won 11 Olympic medals in total – seven gold, two silver and two bronze.
Larisa Latynina, who won 18 medals for the Soviet Union, is the most successful female gymnast in Olympic history.
She was not upset with herself for failing to get closer to that record, though, telling reporters: "I'm not upset about my performance, I'm actually happy, proud and even more excited that it's over, the stress of it.
"I've accomplished way more than my wildest dreams, not just at this Olympics but in the sport, so I can't be mad at my performances.
"I'm pretty proud of myself and it's always so exciting to compete."
Biles' compatriot Jordan Chiles took bronze in the floor event following a successful appeal to her score, after which the two Americans bowed to Andrade on the podium.
"First, it was an all-black podium so that was super exciting for us but then Jordan was like 'should we bow to her?' and I was like, 'absolutely'," Biles said.
"She's such an excitement to watch. It was just the right thing to do."
Biles wins third gold in Paris with emphatic vault victory
American star Simone Biles collected her third gold medal of the Paris Olympics as she triumphed in the vault final on Saturday.
Biles took her overall Olympics tally to seven gold medals as she recorded an average score of 15.300.
The 27-year-old, who withdrew from several events, including the vault, at Tokyo 2020, is now the third most decorated female gymnast in the history of the Olympics, with 10 medals to her name.
She will take part in the floor and beam finals on Monday, meaning she could overtake Vera Caslavska's tally of 11, won between 1960 and 1968.
Larisa Latynina, with 18 golds, is the most successful female gymnast in Olympic history.
Rebeca Andrade of Brazil, who won gold in Tokyo three years ago, claimed silver, while Biles' team-mate Jade Carey took bronze.
Biles claims sixth Olympic gold medal after glittering all-round final display
Simone Biles claimed her sixth Olympic gold medal following a thrilling conclusion to the gymnastics individual all-round final in Paris on Thursday.
After spearheading the United States to gold in the women's team event, she retained the title she won eight years ago in Rio, edging team-mate Sunisa Lee.
Biles' nine medals (six gold, one silver, two bronze) are two better than Shannon Miller, who won two golds, two silvers and three bronzes across the 1992 and 1996 Games.
"It is crazy I am in the conversation of the greatest of all time," Biles said. "I just think I’m still Simone Biles from Spring, Texas, who loves to flip."
Brazil's Rebecca Andrade finished second on Thursday to get silver and Lee took the bronze in the first Olympic matchup of past all-around champions.
Lee became the first gold medalist to return to the podium in the next Olympics since Romania's Nadia Comaneci won gold and then bronze in 1976 and 1980.
Biles started her quest for gold by scoring 15.766 on the vault, almost half a point better than the score she posted in the qualifying round on Sunday.
The American would relinquish her lead to Andrade after her slight mistake on the uneven bars, with Algeria's Kaylia Nemour posting a rotation-best 15.333.
But the reigning world champion would soon be back on top, producing an error-free routine on the balance beam to earn a score of 14.566, handing her an advantage heading into the floor exercise.
And as she did on Tuesday, Biles showed no signs of pressure and performed a routine packed with a series of stunning tumbles to clinch gold at a canter.
The win made her just the third woman to win two all-around golds and the oldest all-around champion since the Soviet Union's Mariya Gorokhovskaya, who won at the 1952 Games in Helsinki at age 30.
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