On a day of generally low scoring in the California desert, the four-times major winner totalled 30 putts on the way to a level-par 72, finishing a distant nine strokes off the first-round lead.
"My timing and rhythm was off a little bit today," Mickelson said after mixing three birdies with one bogey and a double-bogey at La Quinta Country Club, one of three venues hosting the pro-am celebrity event.
"My putter was atrocious and I've been putting great (in practice) too, so that was disappointing. I know that it's there even though the results today were not good. I'm looking forward to playing tomorrow.
"I birdied the last hole and hit some decent shots coming in but (going) out-of-bounds on 10 and (missing) four, five, six-footers, that has prevented me from having a good round today."
Tournament champion in 2002 and 2004, Mickelson three-putted for a disappointing par five after hitting a superb five-wood to 35 feet at the long 13th and drove out-of-bounds at the 10th to record a double-bogey.
"To three-putt (13), it was just a lot of rough stuff today," the former world number two said. "But coming in here, even though I hadn't been able to practise a lot, when did I play and hit balls I was playing great.
"And then today my rhythm was really off. I hit some shots I hadn't been hitting, but kept the ball in play off the tee. I hit a lot of fairways today and was able to shake out an even par round today.
"As bad as that is, I've got a low round in me tomorrow, I think, and it doesn't feel like it's far off."
Asked how he felt during the opening round after struggling with a heavy dose of flu during his tournament build-up, Mickelson replied: "I've felt a lot better. I took it easy on Monday and Tuesday.
"It was like day 10 but I'm fine now. I'm 95 per cent better. I'm not contagious. The energy's coming back, I've been able to practise, I played a little bit yesterday and it's fine."
Mickelson, the leading all-time money winner at the 72-hole pro-am celebrity event which was previously known as the Bob Hope Classic, said he always felt a little anxious before starting his US PGA Tour season.
"When I'm not playing for a while, I really get excited to get back out and I was really amped up," he added.
"I was nervous today and excited to play and maybe a little over amped because again everything was quick - my rhythm, timing, everything was quick. My putting was a little firm.
"So hopefully I'll settle down tomorrow and play a good round."
- Sports & Recreation