Mickelson shot a 1-under 71 on Thursday in the Humana Challenge in his first event since the Ryder Cup, leaving him eight strokes behind leader Michael Putnam.
"Even though this score is the worst I've had in a long time, in months, I'm excited about my game and getting back out tomorrow," Mickelson said. "I can't wait to get started again, because I just feel like I played a little tight today, kind of steered it a little bit."
Adams had a 64 on PGA West's Jack Nicklaus Private Course in his return from hip replacement surgery. He last played a PGA Tour event in March and had surgery in July.
"Very, very blessed to be back out here," Adams said. "It's been a long road to get back. You never know what you're going to get when you have total hip replacement."
The 39-year-old Adams had six straight birdies from No. 14 to No. 1, added three in a row on Nos. 5-7 and closed with a bogey. Playing on a major medical extension, he has 16 events to earn $497,044 to keep his tour card.
"If the putts fall, great, if they don't, so be it," Adams said. "I had a great day today. I'm going to go out there tomorrow and do my best and if I shoot 64 again, great. If I shoot 74, so be it."
Mickelson opened at tree-lined La Quinta Country Club. He made a 12-foot birdie putt on the par-3 third, holed out for birdie from a greenside bunker on the par-4 fourth and made a 15-foot birdie putt on the par-5 sixth. After dropping shots on Nos. 12, 14 and 16, Lefty hit a wedge to 2 feet for birdie on the par-4 17th.
"It was the first round in four months and I'll loosen up," Mickelson said. "Hopefully, the way I'm playing will show in the score. But today, it just didn't."
The 44-year-old Mickelson, the 2002 and 2004 champion, is winless in 27 PGA Tour starts since the 2013 British Open. He has lost weight and gained strength working with trainer Sean Cochran during the long break from competition.
"My body hasn't felt this good in years," Mickelson said.
Putnam made nine birdies in an 11-hole stretch in his 63. He birdied Nos. 7, 9 and 10 on the Nicklaus course and ran off six in a row on Nos. 12-17.
"The course is in perfect shape," Putnam said. "Greens are good. ... I made a lot of birdies at the end. Hit a lot of close shots, a lot of 5-, 10-foot putts. I made them all, so I got to be happy about that."
Mark Wilson, John Peterson, Francesco Molinari and Scott Pinckney matched Adams at 64. Wilson and Peterson played the Nicklaus course, Molinari opened on the Arnold Palmer Private Course and Pinckney was at La Quinta.
The 40-year-old Wilson won the 2012 tournament for the last of his five PGA Tour titles.
"I came in feeling OK, nothing great about the game, but it was ideal today," Wilson said. "I made some saves when I needed it and capitalized on my good shots."
Defending champion Patrick Reed had a 65 at La Quinta. He's coming off a playoff victory over Jimmy Walker two weeks ago in Hawaii in the Tournament of Champions. Last year, Reed shot three 63s to open a seven-shot lead and closed with a 71 for a two-stroke victory.
"I feel like I left a lot out there, but it's always good to shoot 7 under," Reed said. "My confidence is as high as it could be and the main thing is just not getting ahead of myself and not really being over-confident where I take too many unnecessary risks."
Playing partner Matt Kuchar, the highest-ranked player in the field at No. 11 in the world, also had a 65. He closed with a bogey.
"This course here, La Quinta, is fantastic," Kuchar said. "Fun course, challenging, tight off the tees and in great shape. The greens are just spectacular."
FedEx Cup champion Billy Horschel, playing alongside Mickelson at La Quinta, also had a 71. He had three bogeys in a four-hole stretch on the back nine.
"Very sloppy," Horschel said. "I didn't hit it as good as I have the last couple days."
DIVOTS: The Nicklaus and Palmer courses each had a scoring average of 70.077. La Quinta was at 70.000. ... Humana is in its final year as the title sponsor and the Palmer and Nicklaus layouts also are dropping out after the event. ... Luke Donald had a 75 on the Nicklaus course in his first career round in the event.