Hunter Mahan beat Rory McIlroy 2 and 1 to win the World Golf Championships Match Play crown on Sunday, denying the Northern Irishman's bid to claim the world number one ranking.
Mahan moved 4 up through 10 holes then held off the reigning US Open champion to become the first American to win the title since Tiger Woods in 2008.
McIlroy and Mahan had reached the title match with semi-final victories on Sunday morning. McIlroy roared back from 3 down to beat England's Lee Westwood 3 and 1, while Mahan never trailed in a 2 and 1 victory over fellow American Mark Wilson.
McIlroy admitted it was tough to bounce back after the tense battle with Westwood - who also had a chance to supplant England's Luke Donald atop the world rankings by lifting the trophy.
"I knew that was going to be the toughest thing for me today, because I was getting myself up for that semi-final match," McIlroy said.
"That's not an excuse," he added. "He was solid as ever. He's a deserving champion."
After McIlroy and Mahan halved the first five holes, Mahan stuck a nine-iron two feet from the pin for a conceded birdie to win the par-three sixth.
That started a run of three straight holes won for the American, who went 4-up by winning the 10th hole, where his approach from 179 yards out nearly found the cup.
McIlroy didn't win a hole until the 11th, where he chipped in for an eagle to lie 3 down.
He rolled in a birdie putt from within eight feet at 14 to trim Mahan's lead to 2 up.
"He's the best player in the world right now," Mahan said. "I knew he'd make a charge, and he did."
After they halved 15 with birdies, Mahan had a chance to close it out at 16, where his putt for the title brushed the edge and didn't drop.
He made the three-footer coming back to halve the hole with McIlroy, whose birdie attempt from 38 feet had stopped less than two feet from the hole.
They were both on the green in two at 17, but McIlroy, needing to win the hole to extend the match, left his 29-foot putt short. Mahan left his birdie attempt from 21 feet about a foot away and McIlroy conceded the tap-in to end it.
"I tried to claw myself back, but I left myself too much work," McIlroy said.
With his victory Mahan will move into the top 10 in the world rankings for the first time in his career. Mahan also added another elite WGC title to his resume, after his triumph at the 2010 Bridgestone Invitational.
He became only the sixth player to win multiple WGC titles, joining Woods, Australian Geoff Ogilvy, Northern Ireland's Darren Clarke, South African Ernie Els and American Phil Mickelson.
McIlroy looked a different player from the one that rallied past Westwood in the marquee semi-final.
After Westwood seized the early advantage, winning three straight holes from the second to seize a 3 up lead, McIlroy won three of the next four to square the match, and won the ninth to make the turn 1 up.
A fortuitous bounce on a cart path at the 11th helped McIlroy halve the hole with a par. He then won the next two holes to go 3 up, sinking a 25-foot birdie putt at 12.
At 13 Westwood's drive into the gallery found its way into the back of a woman spectator's sweater as she ducked to protect her head as McIlroy found the green.
Westwood pulled one back at 15 - the first hole he'd won since the fourth, but two holes later McIlroy closed it out with a par at 17.
Mahan won the first two holes of his semi-final and never trailed en route to his victory over Wilson.
In the consolation final, Wilson bounced back to beat Westwood 1 up.