Rory McIlroy carded four bogeys and dropped two shots on the fifth as he tumbled down the pack at the AT&T Pebble Beach Pro-Am.
The 28-year-old followed his opening round of four-under 68 with a disappointing 74 to sit tied for 87th on one under, 11 shots behind joint leaders Dustin Johnson and Beau Hossler.
Things rapidly unravelled for McIlroy at the par-4 5th - after a strong drive to the edge of the green, he then missed a lengthy eagle and shorter birdie putts before going on to find the hole on his fifth attempt.
Two further dropped shots on the 8th and 9th signed off a miserable round.
Shane Lowry started brightly with two birdies in his first six holes before dropping three shots between 8 and 12.
A further bogey on 15 would have had him level with McIlroy but a final-hole birdie pushed the Offaly man up to three under and a share of 57th.
Paul Dunne and Seamus Power are both level par after carding 72 but Padraig Harrington is down the depths of the leaderboard as he followed an opening-day 75 with a 76.
Graeme McDowell had three double-bogeys in a disastrous 80 that left him just one shot clear of Harrington on seven over.
Americans Johnson and Hossler, a PGA Tour rookie, share the top spot on 12 under after carding rounds of 64 and 67 respectively.
Johnson hit five birdies on the front nine and followed up with three on the back nine, the only blot in his book being a bogey on the par-4 15.
Hossler didn't drop a shot in his round and is still very much in contention at the half-way stage.
Among the chasing pack are Troy Merritt, who followed his 67 on Thursday with the same score on Friday, while Phil Mickleson went six under to jump 27 places into tied 5th.
Defending champion Jordan Spieth had a better second day with a five-under round of 66 following his opening day 72.