Dustin Johnson birdied two of the last three holes for a one-shot victory at the St. Jude Classic in Memphis, Tennessee, on Sunday while Rory McIlroy squandered a shot at winning with an errant tee shot on his final hole.

In just his second tournament back following almost three months out of the game with a back injury, Johnson shot a four-under 66 to finish at nine-under 271, one clear of fellow American John Merrick (68), to claim his sixth PGA Tour title.

Ryan Palmer (66), Chad Campbell (68), Australian Nick O'Hern (69) and U.S. Ryder Cup captain Davis Love III (69) shared third at seven-under 273.

It was another rollercoaster day for world number two McIlroy of Northern Ireland, who arrived at the 18th hole tied for the lead only to send his tee shot into a water hazard and end up with a double-bogey, dropping him to a tie for seventh.

Johnson started the final round one shot back of overnight leaders Merrick, O'Hern and Love III but fired a laser approach inside four feet on the opening hole and converted the birdie to join the lead.

Back-to-back birdies on the sixth and seventh holes, the first via an impressive 17-foot putt and the next coming from yet another close wedge shot, ensured the 27-year-old stayed on top of the leaderboard.

But when he failed to get up and down from the greenside bunker on the ninth hole and posted bogey, his momentum stalled and a handful of challengers leapfrogged him on the board.

Six straight pars were enough to keep the big-hitting American in the mix before he made his move, just as those ahead of him faltered.

Johnson birdied the par-five 16th to join the lead pack at eight-under and as McIlroy and Campbell sent their tee shots on the 18th into the water, Johnson drained an eight-foot birdie on the 17th to take the outright lead at nine-under.

A routine par on the last was good enough although Merrick, playing in the final group, almost chipped in to force a playoff.

(Reporting by Ben Everill in Del Mar, California; Editing by Frank Pingue)