Returning to PGA Tour play after withdrawing from Torrey Pines last Friday night because of back pain, Mickelson opened his title defense with an even-par 71. That left him seven strokes behind leaders Bubba Watson and Y.E. Yang at Scottsdale, Ariz.
Mickelson said: “My back is fine. My game was a little rusty.”
He had five birdies, three bogeys and a double bogey at TPC Scottsdale.
Mickelson first felt soreness in his back two weeks ago in Abu Dhabi, United Arab Emirates, and pulled out of his hometown event near San Diego after making the 36-hole cut.
Mickelson had a double bogey on the par-three 12th — his third hole of the morning round — after hitting into the left-side water. He made 20-foot birdie putts on the next two holes but three-putted for par on the par-five 15th after hitting a hybrid pin-high from 245 yards.
Watson and Yang shot 64.
Watson birdied four of the final six holes. The 2012 Masters champion had eight birdies and a bogey in the afternoon session. Yang birdied the final two holes. The 2009 PGA winner also had eight birdies and a bogey, playing the back nine in six-under 30 in his morning round.
McIlroy's 63 leads at Dubai
Rory McIlroy took another step toward regaining his form with his lowest score in more than three years, a nine-under 63 while playing with Tiger Woods to take a two-shot lead in the Dubai Desert Classic in the United Arab Emirates.
McIlroy played bogey-free in ideal scoring conditions on the Maglis course at Emirates Golf Club, picking up six birdies and an eagle during a 10-hole stretch in the middle of his round. He easily upstaged Woods, who was five shots behind after a 68.
Edoardo Molinari made an eagle on the par-four second hole on his way to a 65. The group at 66 included Stephen Gallacher, the defending champion who also played in the feature group with Woods and McIlroy.
McIlroy had his best score since a 63 in the Hong Kong Open in November 2010.
Woods managed to hit only half of the fairways in regulation, but taking only 25 putts helped him. The world's No. 1 player also made his first birdie of the year on a par five, his opening hole at No. 10. Woods did not birdie any of the 12 par fives he played last week at Torrey Pines, where he missed the 54-hole cut.