The ROC, whose men’s team won the Olympic gold medal last night, led from the start and survived two falls on the balance beam to close out a victory following a dramatic evening here.

Biles, widely considered the greatest gymnastic of all time, was expected to lead the US to a successful Olympic title defence despite a qualification performance which was uncharacteristically shaky and failed to live up to her high standards.

The 24-year-old multiple world and Olympic champion struggled to land her opening vault in the final and received one of the lowest scores of her career on the apparatus before leaving the floor with a trainer.

Speculation grew that Biles would not return and although she reappeared, she played no further part and was forced to cheer on her team mates from the sidelines.

Biles said she had been "dealing with some things internally that will get fixed in the next few days", adding that she would compete in the individual all-around final on Thursday (July 29).

"I just don't trust myself as much as I used to," she said.

"And I don't know if it's age - I'm a little bit more nervous when I do gymnastics. 

"I feel like I'm also not having as much fun and I know that.

"I have to do what’s right for me and focus on my mental health and not jeopardise my health and well-being.

"That’s why I decided to take a step back."

The ROC, whose team of Liliia Akhaimova, Viktoriia Listunova, Angelina Melnikova and Vladislava Urazova finished more than a point clear of the US in qualifying, ended the competition on 169.528 points, more than three clear of the reigning champions in second.

Britain quietly went under the radar and leapfrogged Italy to seal a superb bronze on 164.096.

There had been signs that not all was well in the US team, which also included Grace McCallum, Jordan Chiles and Sunisa Lee, before the final after they made six major mistakes in qualifying. 

They had made a nervy start even before Biles pulled out of her planned vault and executed an effort with a much lower difficulty, which scored 13.766 - her lowest ever at the Olympics.

The shock early departure of Biles, the reigning Olympic champion in every discipline except for the beam, stirred the US into life but they were unable to keep pace with the ROC.

The ROC team had a healthy lead at the halfway stage but it was cut to 0.800 heading into the deciding floor exercise after an error-strewn rotation on the beam.

Without Biles, chances of the US overhauling the ROC were slim and the "neutral" team finished comfortably clear, with Melnikova sealing the deal with a floor routine that yielded 13.966 points.

"We still can't believe it's happening," she said.

"The impossible is possible now. 

"Of course are very happy.

"We worked extremely hard, we were basically in a closed training camp for a year and a half, working for this moment.

"I knew that it was depending on me and I was feeling overwhelming happiness and I knew I did it. 

"I knew I had done my job."