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Dustin Johnson is more than happy to worry about The Olympic Club and the U.S. Open once he arrives in San Francisco. For now, he's happy to celebrate snapping back from nearly three months off the PGA Tour.

Johnson won the St. Jude Classic on Sunday in his second event back after a back injury cost him nearly three months away from the PGA Tour, shooting a 4-under 66 that held off John Merrick by a stroke. It was Johnson's sixth career victory as he prepares to head to California to compete in the U.S. Open after tying for 23rd last year at Congressional.

"I wasn't worried about the Open ...," Johnson said with a smile. "I was worried about winning the FedEx. Next is a whole different week. It has nothing to do with this week. I'm going to go out and get a game plan together to play that golf course and stick with it. Hopefully just give myself a chance to win on Sunday."

Johnson collected the winner's check of more than $1 million by lowering his score each round, finishing with 66 that was the lowest final round by a winner here since Brian Gay in 2009. He finished four birdies and a bogey, turning in two straight birdies on Nos. 16 and 17 to move atop the leaderboard on a rollercoaster day where as many as eight players had at least a piece of the lead at the TPC Southwind course.

His 9-under 271 total made him the first player not named Tiger Woods to win at least one title in each of his first five seasons out of college. His six career wins are the most of any of the tour players in their 20s.

"It means I'm playing good golf too," Johnson said. "I come out every week and I try to just put myself in position to have a chance to win on Sunday."

And his latest win came after Johnson pulled a muscle in his lower right back in March lifting a jet ski at his home. Rest was the best treatment, and he didn't swing a club for six weeks before returning to the tour last week at Memorial where he finished tied for 19th. In Memphis, he ranked third driving the ball and tied for fourth hitting greens in regulation.

"Well, it feels really good, especially having so much time off," Johnson said of his win.

Merrick remains winless in 154 career tour starts. He shot a 69, recovering from a double bogey where he hit twice into water with a pair of birdies, and had a 272 total. But only he and Davis Love III, the U.S. Ryder Cup captain, carded rounds in the 60s in each round here.

"The wheels were coming off the train. Hung in there and made a good putt for double," Merrick said. "Hung in there, proud of myself."

Love shot a 69 and tied for third with O'Hern (69), Campbell (68) and Ryan Palmer (66) at 273. McIlroy, who now heads to San Francisco and The Olympic Club to defend his U.S. Open title, had a 69 and tied three others at 274. Palmer just missed a trip to the U.S. Open with his finish.

For a man who had missed three straight cuts coming to Memphis, McIlroy said he feels well prepared going into the U.S. Open after playing four straight rounds. He had a two-stroke lead all to himself after four birdies through 11 holes and appeared headed to the win. But McIlroy finished with two bogeys then his first double bogey in two trips to Memphis on No. 18 after hooking his 3-wood off the tee just a bit too much.

"I had a great chance to win this week, and it didn't quite happen," McIlroy said. "So I can take a lot of positives from this week into the U.S. Open next week."

He wasn't alone in finding the lake left of the 18th fairway.

Campbell also hit his tee shot into the lake on his way to bogey. Merrick and O'Hern went to the tee in the final group needing a birdie to catch Johnson, and O'Hern also put his tee shot into the water.

Johnson took control with his consecutive birdies, rejoining the lead pack at 8 under when he rolled in an 11-footer on the par-5 16th. Johnson then moved to the top of the leaderboard by himself at 9 under by hitting his second shot from 161 yards to 9 feet on the par-4 No. 17 before sinking the birdie putt. Johnson said he thought he might have been tied and made sure he hit the fairway to at least give himself a chance at birdie.

Par was good enough to clinch the win in Johnson's first trip to Memphis.

"I will be back," Johnson said.

Tour officials pushed tee times up more than three hours and sent players off both tees in threesomes to avoid thunderstorms predicted for Sunday afternoon that wound up missing the course. With so many bunched closely together, players moved up and down and back up again before Johnson made his final move.

Divots: FedEx sponsors this event, which benefits the St. Jude Children's Research Hospital. ... Johnson is the seventh player in his 20s to win on tour this year. ... This was the third time since 2005 that the winner of this event did not reach double digits under par. Jeff Maggert won at 9 under in 2006 and Justin Leonard won at 4 under in 2008. ... John Daly matched the low round of the tournament with a 64 on Sunday and finished tied for 19th. It was his lowest round on tour since a 64 in the final round of the 2010 Wyndham Championship.