Justin Thomas held off a threat from defending champion Brooks Koepka to win the WGC-FedEx St. Jude Invitational and claim the number one ranking.

Thomas carded a five-under-par 65 in Sunday's final round to soar to the top of the leaderboard and he stayed there for his third title of the season – the most on the PGA Tour.

Former world number one and 2018 champion Thomas was four shots off the pace when he teed off on day four at TPC Southwind, where Brendon Todd was the player to beat overnight in Memphis, Tennessee.

But Todd could only manage a fourth-round 75 and a share of 15th as Thomas – fuelled by a flawless front nine – used six birdies and a solitary bogey to clinch his 13th career crown at 13 under.

On top of the world.

After winning the @WGCFedEx ... @JustinThomas34 is No. 1 in BOTH the #FedExCup and OWGR. pic.twitter.com/GJLvXfzGjO
— PGA TOUR (@PGATOUR) August 2, 2020

The result means Jon Rahm's brief stint atop the world golf rankings is over, with the Spaniard (66) replaced by Thomas after languishing in a tie for 52nd ahead of the US PGA Championship, which starts in San Francisco on Thursday.

Koepka – eyeing back-to-back successes at the tournament – was within one stroke of the lead but the four-time major champion double-bogeyed the last to sign for a 69, having also bogeyed the 16th.

The US PGA Championship holder ended the World Golf Championships event tied for second alongside Phil Mickelson (67), Daniel Berger (65) and Tom Lewis (66).

How many wins this season now, @JustinThomas34?

pic.twitter.com/0yCNEpGWEw
— PGA TOUR (@PGATOUR) August 2, 2020

Xander Schauffele (66), Jason Day (67), Shane Lowry (67), Chez Reavie (68), Louis Oosthuizen (68) and Matt Fitzpatrick (68) finished four strokes behind Thomas, while Dustin Johnson (67), Webb Simpson (68) and Byeong Hun An (73) were a shut further back.

It was another tough outing for Jordan Spieth – the three-time major winner still searching for his first top-10 performance since the Charles Schwab Challenge.

A final-round 71, consisting of two double bogeys, two bogeys and five birdies, saw Spieth slide to four under and nine strokes adrift.

Another former world number one, Rory McIlroy, did not fare much better after finishing tied for 47th following his three-under-par 67.