England's Paul Casey shot a second successive six-under 66 to take the halfway lead at the Memorial Tournament in Ohio.
But it was a day to forget for Rory McIlroy, who was brought crashing back to earth as he carded a six-over-par 78 to tumble down the leaderboard at Muirfield Village.
McIlroy followed up his BMW PGA Championship victory at Wentworth on Sunday with a superb 63 in Thursday's first round to open up a three-shot lead, but the Northern Irishman fell apart on Friday, shooting 15 strokes higher.
The damage was done on a disastrous opening nine where three consecutive double bogeys saw him go out in 43.
Starting on the back nine, McIlroy bogeyed his opening hole, the par-four 10th, before parring 11 and picking a shot back up on 12.
It went rapidly downhill after that though, as double bogeys followed on 13, 14 and 15 before two pars and a further bogey saw him reach the turn four shots off the lead.
That represented a 12-shot collapse on the back nine from Thursday where he had picked up two eagles, three birdies and a double bogey to record a 31.
Nine shots back
McIlroy did manage to steady the ship on his back nine, picking up birdies on the second and seventh, while only dropping a single shot, but it still meant he ended the day nine off the lead having started it three in front.
Casey, who was joint second after a 66 on Thursday, picked up where he left off with an eagle and two birdies in his opening six holes on Friday.
The 36-year-old, who started on the 10th, dropped a shot on 17 but picked up further birdies at 18, 1, 5 and 7 before bogeying the last to finish on 12-under.
Masters champion Bubba Watson threatened to reel in Casey with a round of 69 which included a hat-trick of birdies from the fifth and a tap-in eagle at the 15th.
But the left-hander bogeyed his final two holes to leave Casey three clear.
American Chris Kirk sits in solo third on eight-under after a two-under 70 that included four birdies and two bogeys, while Hideki Matsuyama (67) and Martin Flores (68) are one stroke further back in a share of fourth.
Former champion Justin Rose missed the cut by a single shot after being awarded a one-shot penalty for a double-hit as he attempted a chip on the 12th hole.