Dustin Johnson fired a seven-under 64 to move into a share for the lead at the AT&T Pebble Beach Pro-Am, where Rory McIlroy slipped down the leaderboard.
The world No 1 mixed eight birdies with a sole blemish at Monterey Peninsula - one of three courses in use this week - to move alongside overnight joint-leader Beau Hossler.
Hossler followed his opening-round 65 with a blemish-free 67 at Spyglass Hill to join Johnson on 12 under, with American duo Troy Merritt and Julian Suri two strokes back in a share of third.
Phil Mickelson, Jason Day and Jon Rahm are in the group of six players three off the pace, while McIlroy struggled to a three-over 74 in an erratic putting display to slip into the tie for 88th.
Beginning on the back nine, Johnson - a two-time winner of the tournament - birdied his opening three holes and cancelled out a blemish at the 15th with successive gains over the next two holes.
Johnson reached the turn in 33 and moved into a six-way tie for the lead with back-to-back birdies from the first, before holing a 20-footer at the fourth
McIlroy reached the turn in level-par at Monterey Peninsula and added a birdie at the par-four first, before taking five putts from just off the fifth green - including four from inside six feet - to rack up a double-bogey six.
The Northern Irishman's issues on the greens continued with three-putt bogeys at the seventh and eighth, as playing partner Mickelson stayed in contention with a bogey-free 65.
Day posted four consecutive birdies around the turn to match Mickelson's score and also get to nine under, with Rahm also three back after a blemish-free 67 at Pebble Beach.
Kevin Streelman, Aaron Wise and Steve Stricker complete the share of fifth, while defending champion Jordan Spieth birdied five of his first seven holes on his way to a five-under 66.
Watch the AT&T Pebble Beach Pro-Am throughout the weekend live on Sky Sports. Live coverage continues on Saturday from 7pm on Sky Sports Golf and 8pm on Sky Sports Main Event.