The bogey-free performance was a far cry from last week when Mickelson was forced to withdraw during the third round in San Antonio after pulling a muscle in his right side
"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."
Mickelson is three shots back of first-round co-leaders Bill Haas and Charley Hoffman, both who finished at 7 under. Hass had five birdies on his back nine, while Hoffman needed just 27 putts to earn a share of the lead.
Keegan Bradley and Matt Kuchar lead a group of five golfers at 6 under. J.B. Holmes, Erik Compton and Jim Renner are also at 6 under, with 10 golfers are at 5 under.
Of the 144 players in the field, 113 finished at even par or under on an overcast and occasionally misty day.
Mickelson and Bradley were part of a threesome with Webb Simpson, who also finished 4 under. All three players in the group navigated the course without a bogey in their morning rounds.
Former world No. 1 Rory McIlroy finished with a 2-under 70.
In Rancho Mirage, Calif., Michelle Wie let a couple of shots slip away late Thursday afternoon in the Kraft Nabisco Championship, leaving her a shot behind Shanshan Feng.
Tied for the lead after playing a four-hole stretch in 5 under, Wie lipped out a 3-foot par putt on the par-3 17th and settled for par and a 5-under 67 after her wedge approach released long on the par-5 18th. Golf
Bill Haas -7
Charley Hoffman -7
Keegan Bradley -6
Matt Kuchar -6
J.B. Holmes -6
Erik Compton -6
Jim Renner -6
Phil Mickelson -4
Mike Weir -1
Daniel Summerhays E