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McIlroy, who began the day at even par, was seven-over after his first eight holes. He withdrew from the tournament after hitting his second shot on his ninth hole, the par-5 18th, into the water for his third wet ball of the day.

He left the grounds without speaking. An hour after McIlroy's withdrawal, his managers released a statement from McIlroy, who said he was bothered by a sore wisdom tooth that did not respond to a pain reliever.

"It was very painful again this morning," he said, "and I was simply unable to concentrate."

He added, "This is one of my favorite tournaments of the year and I regret having to make the decision to withdraw, but it was one I had to make."

It was a markedly different ending than last year, when McIlroy staved off a late charge from Tiger Woods to secure his third tour title and the No. 1 ranking. He went on to win three more events, including his second major at the P.G.A. Championship, and Player of the Year honors.

It was McIlroy's third tournament since making the switch from Titleist to Nike and the early returns have not been promising: he missed the cut in his season opener at Abu Dhabi in January, and last week he lost in the first round of a World Golf Championships match-play event outside Tucson.

Before the tournament began, McIlroy, 23, said he was not concerned by his slow start.

"It's not like I'm pushing for answers or I'm looking for answers," he said. "Everything's there. It's just a matter of putting it all together."

McIlroy, who was playing with former champions Ernie Els and Mark Wilson, finished his first round with a bogey on 18 and picked up where he had left off by making a double bogey 6 on his second hole Friday. His round unraveled on the 434-yard par-4 16th, a dogleg right with water on the right. He hit two shots in the water and made a six-foot putt to salvage a triple bogey.