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.
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
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.
McIlroy had his best score since a 63 in the Hong Kong Open in November 2010.