Already struggling with a mystery illness that has left him sick and exhausted after the last two rounds, the last thing Casey Stoner needed was a huge highside at the end of Saturday's qualifying session at Laguna Seca.

The Ducati star was holding second behind Jorge Lorenzo at the time of his fall, and lost the position to world championship leader Valentino Rossi while he was being carried out of the gravel trap by marshals.

Fortunately, Stoner was soon officially declared as 'OK', although he now fears he may have hurt his groin.

"To be honest the bike had given us a few warnings in a lot of places around the circuit today and I'm sure there were a lot of other guys in a similar situation," said the Ducati star.

"The tyre is quite hard on the right-hand side and needs a couple of laps to warm up properly. Unfortunately on the lap before the crash, in the most important corner for heating that side of the tyre up I was held up by two riders having a disagreement and then I got stuck behind one of them into the next right-hander too.

"I was wary that the tyre would be cooler than I needed it to be on the next lap so I went into turn three a lot more carefully than normal, but as I closed the gas on the way into the corner the rear just came around.

"I tried to pick the bike up and go straight into the gravel but it didn't want to and that was it - it threw me straight up in the air! It hurt when I landed but I thought I'd only winded myself - now it looks like I might have torn something in my groin, which is not the best thing to happen before the race.

"Anyway, we have a great bike for this track so we'll see what we can do," Stoner concluded.

Lorenzo fell for the second time just moments before Stoner and has been diagnosed with a separated shoulder and bruised foot. Lorenzo also blamed a cold rear tyre.

Stoner is presently four points behind Lorenzo and nine behind Rossi in the world championship.