JEJU (South Korea) • Three-time Major champion Brooks Koepka was left "heartbroken" after one of his tee shots at last month's Ryder Cup struck a spectator, leaving her blinded in one eye.

But after overtaking compatriot Dustin Johnson and England's Justin Rose to become world No. 1 for the first time with a four-stroke victory at the CJ Cup yesterday, the American admitted he was now on cloud nine and that reaching the summit was beyond his "wildest dreams".

"To be world No. 1 is something I dreamt of as a kid and I don't think this one's going to sink in yet," said Koepka, whose final round of eight-under 64 at the PGA Tour event on the South Korean resort island of Jeju was highlighted by a chip-in for birdie on the 16th.

"I can't even put it into words. I don't think I ever thought I'd be the best golfer in the world. I always wanted to earn my way to No. 1. Everything I've done this year, it's been working."

The PGA Tour Player of the Year carded a total of 21-under 267 to capture the US$9.5 million (S$13 million) tournament, holding off fellow American Gary Woodland, who poured in 11 birdies as he charged to a nine-under 63 for a 271 aggregate.

The US Open and the PGA Championship winner started the week ranked third behind fellow Ryder Cup players Johnson and Rose, and had to shake off some shaky moments early on at the Club at Nine Bridges.

And Koepka, whose win came on his first start of the new PGA Tour season after he skipped the opening two events, showed both sides of his game, hammering out thunderous drives and exhibiting a deft touch around the greens.

He began his final round with a four-stroke lead over fellow American Scott Piercy and England's Ian Poulter.

But it was Woodland who emerged as Koepka's nearest challenger after starting five shots adrift. His four consecutive birdies from the sixth put him into a share of the lead at 14 under, but he blinked first on the inward half with a bogey at the 10th.

Koepka birdied that hole a few minutes later to reopen a two-shot lead, and though Woodland bounced back with two straight birdies at the 12th and 13th, he coolly matched the feat.

After Woodland bogeyed the 17th to fall three strokes behind, Koepka closed with an eagle on the last to cap off the victory in style.

"I just hung in there. I'm not somebody that's going to panic if things go the wrong way, pretty sure everybody can tell that," he added.

"I just wait for my holes, I know I'm going to have some good looks and when I do, you've got to capitalise on them."

AGENCE FRANCE-PRESSE, REUTERS