Humble, TX (Sports Network) - As Louis Oosthuizen tumbled down the leaderboard Sunday, bogey after double-bogey, Hunter Mahan remained steady.

That, along with a few key birdies, was enough for Mahan to hold on for victory on Sunday at the Shell Houston Open.

Mahan carded a one-under 71 to finish at 16-under-par 272, one stroke ahead of Carl Pettersson (71). Oosthuizen began the round with a two-shot advantage, but stumbled to a three-over 75 and took third at 14-under 274.

Mahan earned his fifth career PGA Tour title and second of the season, following his WGC - Accenture Match Play championship in February. He became the only multiple winner on tour this season, and next week will go for his first major title at The Masters.

"I've been kind of close to [winning] here a couple years, boy it feels great to get the W," Mahan said. "Kind of a different situation for me, having the lead for the back nine. To hang on and hit some clutch shots and play 18 well was big."

Because of the victory, Mahan will head into The Masters as the United States' top-ranked golfer in the world, at No. 4, as well as the FedEx Cup points leader.

He has twice finished in the top 10 at Augusta, but missed the cut last year.

Phil Mickelson, the 2011 champ and a three-time Masters winner, shot 71 to share fourth place with Jeff Overton (68), PGA Champion Keegan Bradley (71) and Brian Davis (74). That group ended at minus-12.

"If I had hit good solid shots, I could have made some putts and I just didn't capitalize on a lot of opportunities to make a move," Mickelson said. "Feel like I played well, though, this week, and I'm excited about the ball striking and driving of the golf ball and my putter, and I'm looking forward to next week."

Pettersson played in the second-to-last group and birdied his first hole to get to minus-15.

Oosthuizen and Mahan followed, but Oosthuizen saw his lead melt away quickly during a round filled with frustration. It started with his first shot, which landed in a divot in the fairway, bounced up in the air, and settled back into place amid a pile of muddy debris.

That prompted him to argue for relief, and after several minutes, he was granted a drop shot with no penalty. The South African went on to par the hole, but his round didn't get easier.

He bogeyed the second after his par putt edged to the right, then successfully navigated a birdie putt over a ridge at the third green to get back to 17- under.

Ahead at the fourth, Pettersson drew within a shot of first by rolling in a nine-footer for birdie.

He soon found himself in the lead thanks to Oosthuizen's first double-bogey, which came at the fifth hole, a par-four. His third shot, from the right fairway, went just three feet and left him 22 yards from the pin. He eventually faced a 21-footer for bogey and nearly holed it, but the putt stopped on the lip of the cup.

After tapping in, Oosthuizen dropped to minus-15, tied with Mahan and a shot back of the Swedish Pettersson.

But Oosthuizen's descent wasn't done. Three holes later, at the par-five eighth, he knocked his second shot into some shrubs and took a penalty stroke. Still faced with a hard lie, he hit into a greenside bunker and two-putted for another double-bogey. He followed that with consecutive bogeys to drop to minus-11.

His playing partner, Mahan, took a much steadier approach. He began his round with eight consecutive pars to stay at 15-under and within striking distance of Pettersson.

The decisive lead change came just before Mahan made the turn. At the par- three ninth, he left himself five feet short of the cup with his tee shot and made the putt to reach minus-16. One hole ahead, Pettersson found a greenside bunker and missed a 19-footer for par.

The bogey dropped him into second place, and Mahan didn't hesitate to extend his advantage. Despite teeing into the rough at the 10th, the Texas resident knocked his second within five feet and sank the birdie try to put himself at 17-under.

He played it safe over the next few holes, but tripped to a bogey at 14, where his drive found a bunker. That trimmed his lead to one over Pettersson, while Oosthuizen had clawed back to 13-under thanks to consecutive birdies from the 12th.

But Pettersson wasn't able to make any birdies on his back nine. At the 18th, his 20-foot birdie try was on line, but stopped about a foot short.

"I gave myself good chances coming in," Pettersson said. "Just a little [mad] I left it short on 18. I had a good chance coming in, couldn't get anything to go."

Oosthuizen, meanwhile, was too far back. All Mahan had to do was par out, and he did. A two-foot par putt at the last secured his victory.

NOTES: Mahan's previous best finish at this tournament was a tie for fifth (2007). He pocketed $1.08 million for the victory...Ernie Els, who needed a victory to automatically qualify for The Masters, posted a two-under 70 to tie for 12th at minus-10.