Simone Biles pulled out of the gymnastics women’s team final at Tokyo 2020 with what was later described as a “medical issue” before the United States went on to claim silver.
Biles left the arena after the vault and was replaced by Jordan Chiles, but returned to support her team-mates.
The 24-year-old said: “I’m just dealing with things internally that will get fixed in the next couple of days.”
The Russian Olympic Committee team won gold, with Great Britain taking bronze.
Biles withdrew having only competed in the first rotation, where she scored 13.766 on the vault – her lowest score on that apparatus ever at the Olympics.
There were no obvious signs of injury following the vault, in which she failed to stick her landing, but she returned from the treatment room appearing to flex her ankle, reported BBC Sport’s John Watson in Tokyo.
A statement from US gymnastics said: “Simone Biles has withdrawn from the team final competition due to a medical issue.
“She will be assessed daily to determine medical clearance for future competitions. Thinking of you, Simone!”
The US were the favourites for the team gold in Tokyo after winning five successive world titles – in 2011, 2014, 2015, 2018 and 2019 – in addition to their back-to-back Olympic gold medals in London and Rio.
They had to settle for silver, with Biles receiving a medal on the podium alongside Jordan Chiles, Sunisa Lee and Grace McCallum.
Biles is the most successful US gymnast of all time and winner of four gold medals and a bronze at Rio 2016.
The 30-time Olympic and World Championship medallist needs four podium finishes in Tokyo to become the most decorated gymnast – male or female – in history.
She has reached all five individual finals and is next due compete on Thursday, when she will attempt to become the first woman to retain the Olympic all-around title since 1968.
Biles has been open about the pressure she feels in Tokyo, saying she feels like she has “the weight of the world on my shoulders at times” before competing in Tuesday’s team final.