MotoGP world championship leader Jorge Lorenzo may miss Sunday's China Grand Prix after fracturing an ankle bone in a crash in practice on Friday morning.
The Yamaha rider sat out the second practice session on Friday but may return for qualifying on Saturday.
"There is still a chance he might be able to ride," said MotoGP doctor Claudio Costa. "It will be a race full of suffering, pain and tears.
"But the riders are heroes and they are capable of doing things like this."
Lorenzo lost control of his 800cc machine in a chicane and was thrown off before sliding to a stop.
He took the impact of the fall on his forearms but suffered no injuries to his upper body.
"The diagnosis shows the violence of the trauma he suffered to both his feet in the crash," added Costa.
"His condition will be re-assessed in the morning before any decision is taken about riding."
The practice session in Shanghai was Lorenzo's first since he had surgery on his right arm to relieve compartmental syndrome following his win in Portugal on 13 April.
The Spaniard, who turns 21 on Sunday, has enjoyed a brilliant debut season in MotoGP with three pole positions and three podiums in his first three races.
Lorenzo is joint top of the championship table with his compatriot Dani Pedrosa, who rides for Honda, on 61 points after three races, ahead of Valentino Rossi in third and Casey Stoner in fourth place.
His injury could be an opportunity for his rivals to steal an advantage, and all three were prominent in both practice sessions.
His Yamaha team-mate Rossi was the fastest rider, posting a best lap of one minute 59.906 seconds.
Rossi was two-tenths of a second faster than 2007 world champion Stoner, with Pedrosa third.
Tech 3 Yamaha's James Toseland was 13th fastest on his first visit to Shanghai, and was happy with his progress.
The Briton said: "The layout is possibly the most incredible I've ever seen and the track is really grippy.
"We've had two positive practice sessions and I'm confident that we can improve further tomorrow to enable me to demonstrate my full potential on race day."
~RS~q~RS~~RS~z~RS~28~RS~)