Justin Thomas returned to world number one by winning the WGC-St Jude Invitational in Memphis.

The American began four shots behind leader Brendon Todd but won by three from Brooks Koepka, Phil Mickelson, Daniel Berger and Englishman Tom Lewis.

Defending champion Koepka birdied 17 to get within one of Thomas but found water on the last to end his hopes.

The closing five-under 65 gave Thomas his 13th PGA Tour title four days before the US PGA Championship starts.

He won the US PGA in 2017, his only major title to date.

"It means a lot, especially with how I felt like I did it," he said. "In the past, I struggled with coming from behind. I got wrapped up in how far I was behind and how many people."

Three weeks ago at the Workday Charity Open, Thomas squandered a three-stroke lead with three holes to play and two bogeys dropped him into a play-off with Collin Morikawa, which he lost.

"I feel like I learned a lot from that," he said after moving up from third in the world rankings, overtaking Jon Rahm and Rory McIlroy, to take top spot for the first time since 2018.

"I just didn't get ahead of myself anytime those last two, three holes, where I felt like my mind was kind of wandering and maybe thinking about winning.

"I basically just told myself to shut up and figure out what you're doing because I could lose this tournament just as easily as I won it."

Koepka bogeyed 16 to give Thomas some breathing space but then made a huge birdie putt on the next to close to within a shot before driving into a stream on 18 on his way to a double bogey.

Thomas' short game on the back nine was imperious, with a number of world-class pitches and high-class putting combining to make birdies and save par.

Overnight leader Todd struggled throughout and finished the day four over to tie for 11th.

Lewis finished in joint third after he managed to follow his 61 on Saturday with a fine 66.

His compatriot Matthew Fitzpatrick finished joint sixth on nine under alongside Open champion Shane Lowry of Ireland while Tommy Fleetwood was on three under.