Casey Stoner, who slipped from second to fourth in Sunday's US Grand Prix at Laguna Seca, will stay in the United States for extensive tests at the Fremont Surgery Center of Dr. Arthur Ting, to try and understand the source of his mystery energy-sapping illness.

Prior to Laguna, Stoner had required medical attention for exhaustion and sickness after both the Catalan and Dutch races.

The symptoms reappeared as early as Friday practice at Laguna Seca, and the Ducati star's hopes suffered a further blow when he was thrown from his machine in Saturday's qualifying session, bruising his upper body and sustaining a groin injury.

The 23-year-old had set a time good enough for third on the grid, and underlined his one-lap speed by topping morning warm-up.

Stoner overtook world champion Valentino Rossi for second place behind Dani Pedrosa early in the grand prix, but by lap 22 of 32 both Rossi and Fiat Yamaha team-mate Jorge Lorenzo had found a way past and were pulling away.

Stoner remained fourth to the end, eventually finishing 12 seconds from victory. The Australian received a round of applause from the Ducati team for his efforts but it was little consolation.

“It has been a weekend to forget. I can't be happy with fourth place even if it has limited the damage to my championship chances," said a frsutrated Stoner. "Today I didn't feel as bad at the end of the race as I have done in the last two but I'm still not right.

“I found it really difficult to keep my concentration over the last few laps and I was in a bit of a daze by the end. My left arm was feeling really tired by the end because of the fact there are so many left-handers here and my left wrist still doesn't have the full range of movement after my operation in the winter.

“I'm really disappointed because without all of these problems we could have done so much better. I want to thank the team for giving me a great bike and also Doctor Claudio Macchiagodena and our physiotherapist Freddie [Dente] for all their help.

“Now we'll focus on trying to find out the root of the problem because racing in this condition is not much fun," added the 2007 world champion.

Stoner's symptoms are similar to Epstein-Barr virus, but that remains one of many possibilities at this stage.