Reigning world champion Casey Stoner may not get a chance to defend his Indianapolis Grand Prix title after a crash-filled day on Saturday.

Last year's pole winner crashed in qualifying, tearing a ligament in his right ankle. Series officials wrote on Twitter that the winner of the last three American races was still being evaluated to determine whether he can race Sunday.

American Nicky Hayden crashed and went flew into the air. Series officials said Hayden regained consciousness and was taken to a hospital for a CT scan.

After Stoner's motorcycle wobbled, he tried to regain control but flew over the handlebars and across the grass. He tried to walk back to his bike, but called a medical team over and was taken away on a stretcher.

Originally, doctors thought Stoner had broken his right ankle - an injury that probably would've ended his chance of becoming the race's first two-time winner.

The 26-year-old Stoner, who is racing for the final time in the US this weekend, could still be cleared to ride Sunday. If he does, he'll start sixth on the 23-motorcycle starting grid after completing the 16-turn, 2.621-mile road course in 1 minute, 39.465 seconds. Hayden's teammate with Repsol Honda, Spain's Dani Pedrosa, won the pole with a time of 1:38.813.

Stoner, a two-time world champ from Australia, already announced he'll retire after this season. He wasn't the only rider struggling to tame Indy's slick track.

With about 8 1/2 minutes left in the 60-minute qualifying session, Hayden had an even scarier crash. The Ducati rider wobbled coming through a turn, then spun sideways, launching into the air and somersaulting across the track before coming to a stop in a prone position.

Without the crash, he would have started eighth after posting a fast lap of 1:39.748.

About four minutes before Hayden's crash, American Ben Spies, the 2010 Indy pole winner, slid off his bike and went rolling through the dirt. He got up, made it back onto the track and will start fourth. Following practice, Spies went to the infield track center for more evaluation.

Earlier, Hector Faubel of Spain flew off his bike during qualifying for the Moto3 race. A spokeswoman for the Bankia Aspar team said the rider went to the hospital with bruised ribs and an injured back.

And Hector Barbera, who crashed during Friday morning's MotoGP practice, is heading home to Spain for more tests after fracturing three vertebrae - two of which were broken in a 2008 crash. Toni Elias, the Spaniard who won 2010 Indianapolis Moto2 winner, replaced Barbera on the No. 24 bike Saturday and qualified 17th for the Pramac Racing Team.

- AP