Sung Kang set a new course record at Monterey Peninsula with a career-low round of 60 to hold a share of lead at the AT&T Pebble Beach Pro-Am in California.
Kang sank nine birdies and an eagle in his 11-under round to hold a share of the lead on 11 under with Japan's Hiroshi Iwata, who shot a 66 at Pebble Beach.
"I feel like I'm living a dream right now," Kang told reporters.
Phil Mickelson, Freddie Jacobson and Chez Reavie are all a shot adrift after the second round.
Mickelson needed only 11 putts on the front nine at Montere Peninsula and when he eagled the 10th with a 40-foot putt to g seven under for the round he was flirting with breaking 60.
But bogeys at the 12th and 18th holes ended any hopes of that and he signed for a six-under 65.
"I thought I was going to shoot a lot lower than I did," Mickelson said. "I didn't expect to play one over coming in. But I'm not going to complain because I made a lot of good putts on the front."
Mickelson will play Pebble Beach on Saturday as he hopes to claim a fifth win in the event.
Justin Rose shot a three-under 68 to lie two shots off the pace at nine under, while Jason Day had a 66 at Monterey Peninsula to get within five shots of the lead.
World number one Jordan Spieth had a round of two birdies and 16 pars to put him at three-under and eight shots back.