Justin Thomas surged past the field on the back nine Sunday to win the WGC-FedEx St. Jude Invitational at TPC Southwind in Memphis, Tennessee.

Thomas went five under on the day to finish at 13 under. Brooks Koepka, the defending champion, double-bogeyed the 18th to finish three shots back in a tie for second place with Daniel Berger, Tom Lewis and Phil Mickelson.

As the round inched toward its conclusion, a playoff looked inevitable. Five golfers briefly held a share of the lead at 11 under. Thomas broke away from the back and solidified his hold on first place as the other four fell by the wayside.

WGC-FedEx St. Jude Invitational Leaderboard

1. Justin Thomas (-13)

T2. Daniel Berger (-10)

T2. Tom Lewis (-10)

T2. Phil Mickelson (-10)

T2. Brooks Koepka (-10)

T6. Xander Schauffele (-9)

T6. Jason Day (-9)

T6. Shane Lowry (-9)

T6. Chez Reavie (-9)

T6. Louis Oosthuizen (-9)

T6. Matthew Fitzpatrick (-9)

Full leaderboard available on PGATour.com

The breakthrough came on No. 15. Thomas used a combination of ball-striking and luck on his tee shot as he somehow dropped his drive just beyond a tree in front of the fairway preceding the green. From there, his approached bounced to within six feet of the hole, and he sank his birdie putt to improve to 12 under.

Thomas got another birdie after his third shot on the par-five 16th hole rolled to a stop three feet from the cup. That set him up with a straightforward putt to build a two-shot edge on Koepka, who bogeyed the hole.

Koepka's fortunes altered significantly over the course of the final two holes.

He sank a 40-foot birdie putt to trim Thomas' lead to one shot and leave the door open for a comeback.

The remaining drama evaporated as Koepka's tee shot on No. 18 sailed left of the fairway and into the pond.

That also put him in a tough position to maintain his hold on second place. He deposited his third shot into a greenside bunker before finally reaching the green in four. A two-putt later, Koepka had to close his tournament with a double bogey.

His poor drive proved to be a costly mistake.

Brendon Todd was the leader through 54 holes, followed closely by Byeong Hun An and Rickie Fowler. None of the three was under par in the final round. An and Fowler both went three over, while Todd's five-over 75 tied for Sunday's worst score.

The PGA Tour now heads to TPC Harding Park in San Francisco for the 2020 PGA Championship. Due to the various scheduling changes related to the COVID-19 pandemic, this will be the season's first major tournament.

Koepka enters as the defending champion after his eight-under final score was good enough to claim the title at Bethpage Black in 2019.