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.
Official statement: "Simone Biles has withdrawn from the team final competition due to a medical issue.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."— USA Gymnastics (@USAGym) July 27, 2021
Thinking of you, Simone! pic.twitter.com/QA1GYHwWTv
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"Simone Biles has withdrawn from the team final competition due to a medical issue," USA Gymnastics tweeted.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."