Australian MotoGP rider Casey Stoner will undergo a series of medical tests in the United States to try and determine the cause of a mystery illness that has left him feeling fatigued.

Stoner has been suffering from the problem for the last month and is now seeking medical help after feeling dizzy at the end of Sunday's US MotoGP in California.

"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," he told the official MotoGP website (www.motogp.com).

"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."

The 23-year-old Stoner said he was also experiencing recurring problems with his left wrist, which he broke in 2003 but needed additional surgery last year because it never healed properly.

"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," he said.

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

Stoner won the world championship on a Ducati in 2007 and was second to Valentino Rossi last year but is currently in third place this season after missing out on the podium at the US MotoGP.

"I can't be happy with fourth place, even though I know that we have performed some damage limitation in terms of the overall standings," he said.

"It's a weekend to forget."