Justin Thomas goes top of the world rankings after winning in Memphis ©Getty Images

Justin Thomas won the World Golf Championships (WGC)-FedEx St. Jude Invitational after a tremendous final round at TPC Southwind, which also sees him move to world number one.

Thomas began the day four shots behind fellow American Brendon Todd, the 54-hole leader, but ended up winning by three shots.

He shot a five-under par 65 - a score bettered only by compatriot Ryan Palmer, who shot a 64 to finish tied for 15th place.

Todd, courtesy of a five-over-par final round in which he failed to make a single birdie, joined him in 15th.

South Africa's Erik van Rooyen, the United States' Kevin Kisner and England's Tommy Fleetwood all shot 65.

Thomas moved from fifth to claim the title after a strong start, with consecutive birdies on the second and the third holes.

Despite a bogey on the 12th, consecutive birdies on the 15th and 16th set up Thomas to win under pressure from Brooks Koepka.

Pushing to catch up with Thomas, Koepka drove into the water on the final hole to finish three shots back in second.

Americans Phil Mickelson and Daniel Berger, plus England's Tom Lewis, were tied for second with Koepka.

Fifth is the lowest position at the end of day three Thomas has won from on the PGA Tour.

At 27 years, three months and four days old, only Jack Nicklaus and Tiger Woods reached 13 PGA Tour wins at a younger age than Thomas.

The victory warms Thomas up perfectly for the Professional Golfers' Association of America Championship - the first major of the year - which begins on Thursday (July 6).

Thomas won the event in 2017.