Published: Dec 10, 2011 4:12 PM EST
Updated: Dec 11, 2011 7:30 AM EST

DUBAI, United Arab Emirates (AP) - U.S. Open champion Rory McIlroy finally ran out of gas.

Afer winning in Hong Kong to keep the European title battle alive with No. 1-ranked Luke Donald, a shaky third-round 71 on Saturday left the 22-year-old McIlroy six shots off the lead at the Dubai World Championship.

McIlroy needs to win the tournament, and hope Donald finishes outside the top nine. Even McIlroy admitted that was unlikely, considering he is six shots behind leader Alvaro Quiros and Donald has moved into fourth spot.

"I'm tired. I'm very tired," McIlroy said. "You know, I don't really want to say that that's the cause of playing the way I did on the front nine because I came back on 4-under on the back nine. Yeah, as I have been saying all week and last week, I'm not 100 percent but that shouldn't stop me from going out and playing good golf."

It sure didn't last week. After complaining of fatigue during two average rounds at the Hong Kong Open, McIlroy came from behind on Sunday to win the tournament. He clinched it in dramatic fashion when he holed a shot from a green side bunker.

Still, there is no ignoring that McIlroy has looked run down at times this week.

On Friday he said the fatigue he'd felt for the past several weeks may be the lingering effects of dengue fever. McIlroy said he got blood test results back Thursday that showed his white blood cell count remained low and that he may pull out of next week's Thailand Golf Championship.

The No. 2-ranked McIlroy believes he picked up a "mild case of dengue fever" when he was playing tournaments in South Korea or China at the end of October and early November. Dengue fever is spread by mosquitoes, generally in tropical areas, and can cause fever, rash and joint pain.

McIlroy, who lost "maybe three or four kilos" when he was sick, said he would see the doctor later Saturday to determine whether he would play the Thai tournament.

Asked about the role his busy schedule in recent months played in his health woes, McIlroy said it had worn him down but that he had no regrets about playing the World Cup in China with countryman Graeme McDowell.

"It was touch and go whether I was going to play. I wanted to play because it was a big deal for me and G-Mac," he said. "It was something we were really excited about most of the year.

"As much as maybe I shouldn't have played, I didn't want to let him down."

McIlroy said he just needed plenty of rest to fully recover.

"The last couple of weeks have taken a lot out of me after not giving myself enough time to recover straight after the illness," he said. "It just sort of hit me in the last sort or 10 days."