The World No.1 is now in prime position for another huge bonus next month having cemented his position atop the PGA Tour's season-long FedEx Cup standings, courtesy of his three-shot win at TPC Southwind in Memphis.

If the big-hitting Koepka can hold on to his FedEx Cup lead through the season-ending Tour Championship in August, he will net a US$15 million bonus prize.

"It's incredible but the year is not done; there are a few more things to finish off and hopefully they can go my way," a determined Koepka said after beating a WGC field featuring 45 of the world's top 50.

Ousting overnight leader Rory McIlroy, Koepka fired a five-under-par 65 to finish at 16 under.

Former US Open champion Webb Simpson (64) was runner up at 13 under, while Victorian Marc Leishman (67) was a stroke further behind in third.

Koepka earned a maiden WGC title and second US PGA Tour victory this year, having won the US PGA Championship in May – his fourth major.

In addition to the US$1.75 million winner's prize for the WGC, Koepka locked up a US$2 million bonus for taking an unassailable lead in the PGA Tour's regular season points race.

RIGHT: Overnight leader Rory McIlroy shot 71 in the final round to drop to T4th. PHOTO: Sam Greenwood/Getty Images.

He also pocketed US$1 million by winning the Tour's Aon Risk Reward Challenge – a season-long competition based on a player's average score across a designated hole at each tournament.

"It's nice; I'm sure I can find a way to spend it," Koepka joked in his winning press conference.

"It's incredible what I have done this year. The overall consistency, I have taken my game to a new level."

Koepka adds a maiden WGC title to a trophy cabinet featuring two US Opens and two US PGA Championships, all of which he has won during the past three years.

"Any time you can win a world golf championship is special," the 29-year-old Koepka said.

"The goals are to win as many times as you can in a year, turn up for the big events and make sure you're in contention."

While Koepka cemented his place atop golf's world rankings, four-time major winner McIlroy was left to rue a Sunday collapse.

The Northern Irishman, playing in the final group with Koepka, was the only golfer in the top 10 on the leaderboard to not break par – shooting a 71 to drop to 11 under and a share of fourth.

After Leishman, Cameron Smith was next best of the Australians at six under, having posted a 68 to earn a creditable tie for 12th place.

Adam Scott signed off in style with a 67 to save a one-under total, which countryman Jason Day also signed for courtesy of a closing 70.

- Evin Priest, Australian Associated Press