A three-times champion at the picturesque Torrey Pines venue which hugs the bluffs above the Pacific Ocean, Mickelson struggled on the greens on the way to a level-par 72 in the opening round.
The American left-hander offset four birdies with four bogeys on the easier North Course and knows he will have to respond with a low score on the tougher South layout in Friday's second round if he is to climb into contention.
"There are some low scores out there if you play the course right and effectively," Mickelson said about 7,698-yard South. "There are probably half the holes where you've got to play for par and the other half you can make some birdies."
Asked if it was possible for a player to win the tournament without scorching the North layout, Mickelson replied: "Sure. It's going to take some exceptional golf over on the South, but three of the rounds (this week) are on the South.
"I've got to shoot something in the sixties, and see if I can get a couple of putts to go (in). I putted very poorly today so I've got to get that putter going. If I can putt well, hopefully, the mid-sixties."
Mickelson, who has twice apologised this week for what he described as "insensitive" and "dumb" comments about soaring tax rates for millionaires in his native California, totalled 30 putts in the opening round.
"I didn't play the best. I hit it poorly. I putted poorly," the four-times major winner said.
"But I love being here at Torrey Pines. The golf course is in great shape, the greens are as firm as I've ever seen them and I'll see if I can get it going tomorrow."
Mickelson, who won the first of his three Farmers Insurance Open titles 20 years ago, felt he had paid the penalty on Thursday for being "very lethargic" on the first tee.
"I've got to get my head a little bit more focused on the shots," the 42-year-old said. "I haven't been as mentally focused starting out, so, hopefully, I'll be able to turn that around tomorrow.
"Something I'm going to be working on tomorrow is getting a better mental picture of the shot I'm trying to hit and create it, trying to hold it longer while I'm creating the swing. I haven't been as sharp as I need to be."
Asked whether he had been criticised by any of the fans at Torrey Pines on Thursday because of his tax comments, Mickelson replied: "Nothing negative. There have been some good positive ones though."
Mickelson hinted after last week's Humana Challenge he was considering making "drastic changes" and might leave California because of soaring federal and state tax rates but has since said he should have kept his thoughts to himself.
"I made a big mistake talking about this stuff publicly, and I shouldn't have done that," he said.
"It was insensitive to talk about it publicly to those people who are not able to find a job, that are struggling paycheck to paycheck."
Mickelson, a 40-times winner on the US PGA Tour, has piled up career earnings of more than $67 million and considerably more via corporate endorsements and his golf course design company.
According to Forbes magazine, he earned $43 million in endorsements in 2012, second only to Tiger Woods among golfers and seventh among all athletes.
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