(CNN) -- Rory McIlroy revealed that he might be battling a tropical disease after his bid to become Europe's top golfer faltered at the end of his second round at the Dubai World Championship on Friday.
The 22-year-old, who needs to win the $7.5 million season-ending event to stop Luke Donald making history by topping both the European and U.S money lists, was left five strokes behind leader Alvaro Quiros after dropping shots at his last two holes.
The world No. 2 carded one-under-par 71 at Jumeirah Golf Estates' Earth Course to be tied for third on seven-under 137 at the halfway stage.
Top-ranked Donald can clinch the Race to Dubai crown with a finish of ninth even if McIlroy wins. The Englishman was tied for 12th on four under as he bounced back from his opening 72 with a 68 that featured birdies at the last three holes.
McIlroy overhauled Quiros to win the Hong Kong Open title last weekend and keep the race going into the final leg, but was similarly beaten by the Spaniard at the Dubai Desert Classic earlier this year.
The U.S. Open champion confirmed that his recent illness might be a mild case of Dengue fever, which is transmitted by mosquito bites.
"I'm going to see a doctor again tomorrow night and she's going to determine whether I go to Thailand next week," he told reporters.
"They said it could have been Dengue fever, food poisoning or a number of different things. My immune system is a little low and it's taking my body a bit more time to recover from it usually would.
"It didn't go so low to say that I had a bad case of it, but to be honest I'm ready for the season to be done and looking forward to putting the clubs away."
Donald, who clinched the PGA Tour money list by winning the final tournament, was relieved after his late surge lifted him up from 31st on the leaderboard.
"Just personally in terms of my mental approach, that was huge," the world No. 1 said. "It would be foolish to me to rely on him (McIlroy) not winning. I know what's at stake and it's big deal to me.
"I was frustrated out there. I tried to stay patient, but it's tough. I want to finish off what I came here to do and those last three brought a smile to my face.
"There's certainly a lot of pressure. I'm still making a few mistakes maybe because of what's at stake. When there's a lot on the line you feel more nervous, but sometimes it's a good thing."
Quiros fired 64 to lead by four shots from Peter Hanson, who held the first-round lead but dropped back with a 72 on Friday.
Quiros bogeyed the 18th in his 68 on Thursday, but an eagle-three this time gave the world No. 52 a healthy buffer over the second -placed Swede and high hopes of breaking into Europe's top 15 -- which will guarantee a share of the $7.5 million bonus pool offered in addition to the tournament prize money.
"Obviously they are fighting for the Race to Dubai final top position, but we are playing here to try to ruin the party," Quiros said of McIlroy and Donald.
"They are not the only important people in the golf tournament."
England's Robert Rock shot 69 to be level with McIlroy, while compatriot Paul Casey carded 66 to be equal fifth with Scotland's Paul Lawrie (73).
South Africa's 2010 British Open champion Louis Oosthuizen made 67 to be tied for seventh on 139 along with Italy's Francesco Molinari (68), Irishman Shane Lowry (70) and Spain's Pablo Larrazabal (68).