But after signing for a 2-under 69, Lefty said he's hitting too many good shots to dwell on two bad holes.
"I hit a lot of really good shots today, hit a lot of good iron shots, had a lot of fun," Mickelson said. "I probably didn't score as low as I felt I played, but I'm very encouraged with the number of quality shots I hit and how my game feels."
The three-time Phoenix Open champion caught fire early with three birdies and an eagle on his opening nine. Mickelson was in the lead for a brief time, but then his round started to fizzle on the par-3 4th, his 13th hole of the day.
Mickelson thought he struck his tee shot perfectly, but it came up short of the green and just outside of a bunker. He had to stand in the bunker with the ball above his feet, and even with all his short-game wizardry, Mickelson chunked his chip shot and made bogey.
On his next tee shot, Mickelson hooked his drive O.B. then three-putted for a triple bogey.
And just like that, Mickelson went from 5 under to 1 under.
"I hit a bad shot at the wrong time," he said. "It's the tightest hole out there. You miss the fairway five yards left, you're in the wash, in the hazard. You miss it right of the bunker, you're out of bounds. I just hit the one bad shot at the wrong time, and it's unfortunate but I'm not going to dwell on the one bad one because there were really a lot of good ones."