Rory McIlroy pulled off a remarkable win over Lee Westwood in Tucson and was left needing to beat American Hunter Mahan to become World Number One for the first time.

The two European stars met in the semi-finals of the World Golf Championships-Accenture Match Play Championship and Westwood, also with the chance to dethrone Luke Donald at the top of the Official World Golf Ranking, went three up after only four holes.

In 60 previous games during the week only two players had come back from such a deficit to go through, but McIlroy became the third with a thrilling display.

Westwood opened the door by bogeying the fifth and asking McIlroy to hole from two feet for par there appeared to fire Northern Ireland's US Open Champion into action.

He birdied three of the next four to turn one up and then had three more birdies in a row from the 11th.

Westwood was able to match only the first of them, so found himself three down. He then produced an 18 foot eagle putt after driving the green on the 343 yard 15th, but missed from similar range at the next and conceded defeat after bogeying the 17th.

Mahan, meanwhile, beat compatriot Mark Wilson 2 and 1 in a game overshadowed by the clash between the second and third seeds.

McIlroy knew Mahan presented a tough hurdle in the 18-hole final, though. He has been a member of the last two American Ryder Cup teams and was the player, of course, whose defeat to Graeme McDowell at The Celtic Manor Resort two years ago gave Europe a nail-biting win.

He was also the only one of the four quarter-finalists who had a world championship to his name and he was never behind to Wilson, who bogeyed the first and failed to match his opponent's birdie four on the next.

Mahan did have one horrible shank at the fifth, but needed only a par to take the seventh and he matched Wilson's two birdies - the first of them a chip-in - at the start of the back nine.

Wilson got back to only one down when Mahan bogeyed the short 12th, but he was bunkered on the 15th and Mahan's seven footer took him two in front once more.

At the end there no hiding how pleased McIlroy was.

"It was great," he said. "It was not the start I wanted. Three down after four is very tough against anybody, but against Lee, who's one of the best in the world, I was just trying to get back to level at some point in the round.

"To turn one up was a bit of a bonus. I made a few birdies and definitely played well to win that one." He was six under for the 17 holes and Westwood three under.

McIlroy added: "Maybe there's a little extra intensity (at three down), but I think you just have to stay patient and chip away."

On being one game away from possibly reaching World Number One for the first time he said: "It's very exciting. After Luke departed early (in the first round to Ernie Els) that was the goal, to give myself a chance."

Westwood said: "Rory played well. I think we both played well - it was just one of those typical match play games that went a lot with momentum."

He felt he was unlucky on the fifth when he flew the green and considered McIlroy even more fortunate at the long 11th, where the Ulsterman's wild second shot was heading for desert and possibly out of bounds, but hit a cart path and finished on grass near the green. It was halved in birdies.

"That sort of thing can change a match. Go back to level there it's a different kettle of fish. I thought it was big turning point.

"But I'm playing great. I've shot four or five under every day and that's pretty good golf. He can't let a game of match play with all its fickleness get in the way of when you are playing well.

"You build up your confidence and carry that forward."

He still had another game to play too. The event includes a third place play-off and there was a lot of money and loads of Ryder Cup points to play for against Wilson.

Winning the final was worth €1,060,767, while there was €644,037 for second, €454,614 for third and €371,268 for fourth.

McIlroy missed a golden opportunity to take the first, missing a four foot par putt after Mahan had already bogeyed, and the second was then shared in birdie fours.

It was a curious start to the final. The second and third were halved in birdies, but the first and fourth in bogeys. McIlroy had to take a penalty drop on the last of those after his drive kicked off a spectator into an unplayable bush, but Mahan failed to get up and down from a bunker and let him off the hook.

The curiosity included the American backing off his short par putt as an umbrella was caught by the wind and flew away from one of the hospitality units.

A deflated-looking Westwood, meanwhile, was two down to Wilson after six.