Los Angeles - Australia's Jason Day celebrated his return to world number one with his second WGC Match Play crown on Sunday, beating South Africa's Louis Oosthuizen 5 and 4 in the title match.
Day was assured of regaining the world No 1 ranking on Saturday when he reached the semi-finals as American Jordan Spieth was eliminated in the round of 16.
And on Sunday, five days after fearing a bad back might force him out after his opening match on Wednesday, Day completed an unbeaten week at Austin Country Club in Texas with a gritty 1 up semi-final win over defending champion Rory McIlroy followed by his victory over Oosthuizen.
"To be able to play the way I did from tee to green and then on top of it make tough matches and hit the clutch shots has been fantastic," said Day, who arrived in Austin off a win last Sunday at the Arnold Palmer Invitational in Bay Hill.
Day joined Tiger Woods and fellow Australian Geoff Ogilvy as multiple Match Play winners.
He won his first title in 2014 in Arizona before the tournament adopted its current round-robin format for the first three days.
Third-seeded McIlroy, denied a chance to join Woods as the only back-to-back winners of the event, then was beaten 3 and 2 in the consolation final by 52nd-seeded Spaniard Rafa Cabrera Bello.
Oosthuizen, the 16th seed, had beaten Cabrera Bello 4 and 3 in the semis, but he was no match for the second seeded Day in the final.
Day opened the title match with a bogey to go 1 down, but squared the match at the third when Oosthuizen conceded the hole in the wake of a wayward tee shot.
Day took a 1 up lead with an 11-foot birdie at the fourth and stretched his lead from there.
He was 3 up through nine, and drained a six-footer for birdie at the 13th to go 4 up.
He landed his approach at 14 within three feet for a birdie to seal the win.
"I'm very, very thrilled. It's been a memorable week this week not only to win the Dell Match Play Championship but also to get back to number one in the world," Day said.
He said the see-saw battle with McIlroy in the semi-final was nerve-wracking, but he drained a 13-foot par putt to halve the final hole and seal the victory.
That match was all square through 11 when Day drove the green at the par-five 12th, winning the hole with a birdie then winning the 13th with a chip-in birdie.
McIlroy drained a 12-foot birdie putt to win the 14th, but couldn't win another hole.
"It was very stressful," Day said. "There were moments where I think they're the most fun, because I have to get up and hit the clutch putt at the right time.
"I wasn't as tight from tee to green as Rory was. I just kept on saying I've got to frustrate him with my short game.
"If I miss a green I've got to get up and down. If I don't hit a good chip then I've got to hole the putt."
McIlroy called it a "good quality match".
"I did miss a couple of opportunities on the front side," McIlroy said. "He got off to a great start. I didn't birdie 12 or 13, that was probably, I feel, what cost me the match.
"From there I was really just battling back."
Day, who confirmed his status as a Masters favourite with the first major of the year coming up in two weeks at Augusta National, said the same sharp short game that carried him to victory at Bay Hill was key in Austin.
But the mind-set for match play versus stroke play was different.
"You're playing against a guy that's across the tee from you," Day said. "You have to know when and when not to go for things. When someone makes a mistake (you must) be able to aim it 20 feet from the pin and be able to hole the putt."
World Golf Championships Match Play scores on Sunday at Austin, Texas (x-denotes seeding):
Jason Day (AUS x2) bt Louis Oosthuizen (RSA x16) 5 and 4
Rafa Cabrera Bello (ESP x52) bt Rory McIlroy (NIR x3) 3 and 2
Jason Day (AUS x2) bt Rory McIlroy (NIR x3) 1 up
Louis Oosthuizen (RSA x16) bt Rafa Cabrera Bello (ESP x52) 4 and 3