Casey Stoner may have been fast in Friday practice for the US Grand Prix at Laguna Seca, but the MotoGP title contender is facing a fitness battle after symptoms of his mystery illness appeared much earlier than at previous events.

The Ducati star has required medical treatment for stomach cramps and sickness after each of the last two rounds, which he has finished in third position, and - despite new medication - was feeling nauseous after just an hour of track action in California.

Nevertheless, Stoner was fastest for much of the session, before being overcome by Fiat Yamaha riders and title rivals Valentino Rossi and Jorge Lorenzo. Stoner finished the day 0.189sec behind Rossi and only 0.077sec behind Lorenzo.

“I had plenty of time to recover between the Barcelona and Assen races but still encountered the same problems, so it's no surprise to come here after just six days and be struggling,” said Stoner.

“The feeling of tiredness and sickness has started earlier than it did in those two race weekends so it's not looking good but again we have found a good set-up with the bike, we know we have something competitive and I just wish I was able to use it more.

“Since Barcelona we've found a geometry set-up that gives me great feeling but I'm not getting the most out of it - I know it can go faster round here. We'll just have to see how the weekend pans out.

“It is hard for the doctors because I don't have any symptoms until I am physically tired, at which point I go rapidly down the other side of the hill, so it's not like they can give me antibiotics because they don't know what it is.

“Anyway, they have given me something new to try here and we'll see if we can bring it back for race day. If it works I'll be very grateful,” admitted Stoner, who lost out to Rossi in a thrilling battle at Laguna Seca last season.

Stoner is already aware that his symptoms are similar to Epstein-Barr. He has been discussing it with a doctor in Australia, who said it could be EB.