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April 3rd 2014, Humble, Texas, U.S.A.: Phil Mickelson is among the contenders after shooting a bogey-free 4-under par 68 in his opening round of the Shell Houston Open, showing no effects of the muscle pull that forced him to withdraw from last week's Valero Texas Open.

The five-time major winner, who practiced at Augusta National for two days this week in advance of the Masters, is three shots back of leaders Bill Haas and Charley Hoffman -- both who shot 7-under 65.

Keegan Bradley and Matt Kuchar lead a group of five golfers at 6 under after Thursday's play. J.B. Holmes, Erik Compton and Jim Renner are also at 6 under, while 10 golfers are at 5 under.

Former world No. 1 Rory McIlroy, playing in the afternoon on the 7,441-yard Golf Club of Houston, finished with a 2-under 70-- five shots back of the lead. World No. 10 Dustin Johnson withdrew after an 8-over 80.

While Haas had five birdies on his back nine to earn a share of the lead, it was Mickelson -- who was forced to withdraw during last week's third round in San Antonio after pulling a muscle in his right side -- who was the center of attention leading into next week at Augusta National.

Mickelson has won the Masters three times, the latest coming in 2010, but his status was in doubt following last week's injury.

"I'm surprised because I was worried about the Masters," Mickelson said. "I was certainly worried about Houston, but it healed a lot quicker. I felt great today; didn't feel any pain or discomfort and didn't even think about it."

The left-hander, however, underwent treatment in the days following his departure from San Antonio -- along with fitting in two days of light practice at Augusta National -- and appeared in top form on Thursday.

Opening on the back nine at the former Redstone Golf Club, which was renamed the Gold Club of Houston following an ownership change, Mickelson was bogey free despite needing 31 putts.

His playing partners, Bradley and Webb Simpson, also were bogey free on the overcast and occasionally misty day.

"The greens are absolutely perfect," Haas said. "... I think the scores are reflective on that. They were a little slower, but part of that, too, is the weather. It was like a little dewy mist out there all day, which played a part in the speed of the greens."

Mickelson made the turn at 2 under following a 16-foot birdie putt on the par-4 18th, and he birdied both of the par 5s on his back nine to put himself squarely in contention. Mickelson won the Shell Houston Open in 2011, and the course's Masters-like setup -- including slick greens and tight rough -- is a key reason he's returned every year since.

"One of the things I really worked on hard today was staying focused on each shot," Mickelson said. "... I needed to play here this week and really challenge myself in that regard to give myself the best chance for next week."

Kuchar, who finished tied for fourth at last week's Valero Texas Open, had three straight birdies in the middle of his round on Thursday -- highlighted by a 47-foot birdie putt on the 445-yard par-4 second hole.

McIlroy, one of five of the top 10 players in the world in the field, was as low as 3 under before a pair of bogeys on the back nine. The Northern Ireland golfer struggled with 31 putts.

Hoffman needed just 27 putts in earning a share of the lead, reaching the par-5 eighth hole in two shots and two-putting to reach 7 under as one of the final afternoon groups.