American gymnast Simone Biles walked away from the Olympic women’s team competition final on Tuesday night, admitting she had mental health concerns.

The biggest star in US Olympic sports pulled out minutes into the event against Russia, after starting on the vault at the Ariake Gymnastics Centre in Tokyo.

The 24-year-old and five-time Olympic medallist, with four golds and one bronze, was expected to clean up the competition and lead America in another dominant performance.

But the entire event was turned on its head in an instant when Biles, the world and Olympic all-around champion, completely mistimed her opening vault to earn a low score of 13.766 – the lowest of all competitors – and pulled out.

She grimaced after her run and appeared to tell her coaches she was done and didn’t want to do it.

Simone Biles stumbles upon landing after competing in the vault. Photo: Getty

US Gymnastics later confirmed Biles would not compete further, citing a medical issue.

“Simone has withdrawn from the team final competition due to a medical issue,” a statement said. “She will be assessed daily to determine medical clearance for future competitions.

“Thinking of you, Simone!”

Speaking after the event, Biles said she pulled out because she didn’t want to jeopardise her team’s chances of winning.

She explained that she had been “fighting all those demons” inside her head.

“It’s just me in my head,” Biles said.

“I have to focus on my mental health and not jeopardise my health and wellbeing.

“We have to protect our body and our mind.

“It’s very unfortunate this has to happen at this stage … it just sucks when you’re fighting with your own head.

“Tonight they (her teammates) get a gold medal from me for fighting.”

She added that her coaches supported her decision.

Simone Biles of Team United States is embraced by coach Cecile Landi during the final. Photo: Getty

“They saw I was going through it and they totally agreed it was not worth getting hurt over something so silly, even though it’s so big – the Olympics Games,” she said.

“But at the end of the day, we want to walk out of here, not be dragged out of here on a stretcher.”

The loss of Biles though was a setback the US never recovered from as Russia, competing as representatives of the Russian Olympic Committee (ROC) because the country was stripped of its flag and anthem for doping offences, claimed the team title for the first time since the Unified Team at the 1992 Barcelona Games.

The US, winners in 2012 and 2016, settled for silver and Britain claimed the bronze.

– with AAP