Mickelson is in a six-way tie for third place following his memorable 65, which saw the American star hit every single fairway for the first time since 1998. "I'm not going to chase them as hard as I tried to maybe force or chase them previous year because you can get hurt doing that".
Jason Day is snapping at the leaders' heels midway through the opening round at the Pebble Beach Pro Am.
Mickelson thrust himself into contention after hitting every fairway for the first time in his career.
"So, if I just think about it that way, it kind of certainly makes me a little happier, frees me up a bit".
Day carded seven birdies in his 65, while two-time champion Dustin Johnson and 2017 victor Jordan Spieth also took advantage of the favourable scoring conditions on the Monterey Peninsula course as they opened with 66s, while last week's Phoenix Open runner-up Branden Grace returned an impressive five-under 67 at Pebble Beach.
"It was a really solid round, good solid start, and we had handsome weather and the golf course here is in spectacular shape", said Mickelson, the world number 29.
"And I think that made a big difference and I think that rule is really turning out to be a good one". "This was a day that we had great weather, the courses were very scorable, you need to take advantage". Korean Ho Sung Choi, whose unique "Fisherman Swing" has made him an internet sensation, shot a 1-over 72 at Pebble Beach in the 45-year-old's first round in a PGA Tour event. He birdied his last hole to reach 10-under par, joining Lucas Glover (66 at Pebble Beach) and Scott Langley (69 at Spyglass Hill). Cody Gribble grabbed a share of the lead as well with a 6-under-par 66 at Spyglass Hill.
Spieth is part of a field that also includes world number three Dustin Johnson, who won the European Tour's Saudi International on Sunday.
Severe rain is forecast for California's Monterey Peninsula during the next three days and PGA Tour officials are bracing for a Monday finish.
Spieth was even at the turn but ran in five birdies coming home.