American favourite Connor Fields crashed heavily in the third run of his semi-final in the Olympic men's BMX race on Friday and was taken off the track on a stretcher.
Fields crashed on the first 180 corner and was treated by medical staff before being carried away and loaded into an ambulance.
An American team spokesperson later said he was "awake" and awaiting further evaluation, although the extent of his injuries were still not clear.
The reigning champion had already done enough to qualify for the final later, but his hopes of a second successive gold medal came to an end in brutal fashion.
He was close to the front heading into the first steeply-banked corner at the Ariake Urban Sports Park and appeared to tangle with another rider, crashing down heavily.
"We can confirm that Connor Fields is awake and awaiting further medical evaluation," a team official told Reuters.
"We will share additional updates as they become available."
The semi-finals had been delayed for 45 minutes by rain and when racing begin there were several big crashes, although the course was dry and did not appear to be a factor.
"I don't think that the track or the weather had anything to do with the crashes," Dutch rider Merel Smulders, who took bronze in the women's race after her sister Laura also crashed in the semi-finals, told Reuters.
"You know, it's the semi-finals of the Olympics and everyone rides with emotion and they take more risks than normal.
"I feel like there were a lot more crashes in Rio. But there were some bad crashes today and no one wants to see that."
Dutchman Niek Kimmann won the gold medal.
(Reporting by Martyn Herman; Editing by Shri Navaratnam and Stephen Coates)
