Canadian Nick Taylor fired a six-under par 66 to take a two-shot lead after the second round at the AT&T Pebble Beach Pro-Am - but a disastrous double-bogey, double-bogey finish saw Graeme McDowell tumble down the leaderboard.
Taylor birdied four of his last five holes to take his first 36-hole lead on the PGA Tour, at 14-under.
Ireland's Seamus Power made his presence felt with a pleasing second-round 68 at Spyglass Hill.
Australia's Jason Day is well-placed in second, after a superb 64 took him to 12-under overall. Day holed a 40-yard wedge for an eagle on the par-five 14th.
Defending champion Phil Mickelson moved up to third after going around in seven-under, leaving him on 11-under after two days.
McDowell was the best of the Irish after Thursday's opening round 70 at Spyglass Hill, but he struggled from the opening hole at Monterey Peninsula just days after he won the Saudi International.
McDowell went out in two-over-par 36 having had three bogeys and a lone birdie on his opening nine. Birdies on holes 14 and 15 had him back level par for his round - two-under for the tournament - but his calamitous final two holes saw him fall back to two-over-par for the week.
Power sits best of the Irish on four-under-par for the tournament after his mostly tidy round of 68 at Spyglass Hill lifted him into a tie for 35th spot.
The Waterford man bogeyed the first and last holes of his round, but did manage to pour in six birdies in between to add that 68 to his opening-round 72 at Pebble Beach.
Ryder Cup captain Pádraig Harrington shot one-over-par 73 at Pebble Beach to sit even par overall.