Jason Bohn fired a 5-under 67 on Friday to take a two-shot lead at 9 under after the second round, while Mickelsons double bogey on Quail Hollow Clubs easiest hole began a horrible closing stretch that left him seven shots back.
Anthony Kims 67 put him at 7 under and alone in second place, while 2006 champion Jim Furyk, Dudley Hart and George McNeill were three strokes back after 67s.
Mickelson, the biggest draw in this event with defending champion Tiger Woods home nursing his surgically repaired left knee, finished with two double bogeys in his final four holes on the way to a 74.
While Mickelson was hitting his ball into the creek on No. 18 for another double bogey, the loose Bohn was joking with reporters.
Bohn, whose sixth-place finish at the Verizon Heritage two weeks ago secured his playing privileges after his injury-filled 2007, followed his 68 Thursday with six birdies and a bogey.
Im just excited to play. I dont have a lot of pressure now that my medical status is taken care of, Bohn said. I can just go out and free swing it.
Mickelsons free swinging got him into trouble, a day after a 68 left him a shot behind first-round leader David Toms.
With an afternoon tee time Friday, Mickelson birdied the 10th hole to get to 6 under. He missed a birdie putt at No. 14, then strolled to the par-5 15th looking to pick up at least a shot with the leaders at 8 under.
The 15th had a scoring average of about 4 1/2 , and is the last good chance to gain ground before the difficult closing three holes.
But Mickelson pulled his tee shot into the left rough. His second shot was even worse, going nearly straight left. While the ball just cleared the water, it nestled in a horrible lie of pine straw at the base of a tree.
Mickelson hacked at his sunken ball, and it dribbled 30 feet. His fourth shot landed in front of the greenside bunker. A chip shot and two putts later, Mickelson had a 7 and had fallen five shots behind after Bohn had birdied the same hole in the group ahead of him.
Mickelson parred 16 and 17 before his waterlogged finish left him with plenty of work to do to win this tournament for the first time.
There was a low round out there and I let it slide, Mickelson said. The last four or five holes, I made a few mistakes that cost me.
Toms, who has been fighting a sore back for two years, couldnt build off his best round this year. The 41-year-old Toms slogged through a 3-over 75 that left him tied with Mickelson for 25th place.
With Toms and Mickelson struggling, Bohn shot ahead of them on the strength of some changes hes made with veteran coach Mike Shannon.
The biggest change for me is that my putter was lengthened an inch, said Bohn, who was held to 17 tournaments last year due to a rib injury. My coach has been wanting to do this for almost two years now. I just didnt feel comfortable.
Finally I wisened up and listened to him.
Bohn will be paired on the final group Saturday with the 22-year-old Kim, who also has taken a different approach. But for Kim, its mental.
A year after he had four top-10s as the PGA TOURs youngest rookie and a third-place finish early this year at the Bob Hope Chrysler Classic, the confident Kim fell on hard times. He missed three cuts before rebounding to finish tied for second at the Verizon Heritage last month.
I just realized that I wasnt putting in the time, and to play against the best players in the world, you have to do that, said Kim, who recently switched to veteran caddy Eric Larsen. I think if I practice hard, anything is possible. I guess I was just talking a little bit more than I was practicing. So this year Ive changed that and Im going to keep riding that wave.
The old-style, tree-lined setup included more wind and faster greens than Thursday, and the scoring was up'minus Jay Williamsons hole-in-one at No. 6.
McNeill had one of the wildest rounds, with six birdies, an eagle and three straight bogeys. Furyk and Dudley, meanwhile, took advantage of their early wakeup calls.
It was really nice to get a morning tee time, said Furyk, who is 1-1 in playoffs at this tournament. I played probably six holes where it was pretty benign out there, where there was really no breeze, a little moisture on the greens. But the wind picked up at that point.
Bohn overcame the obstacles during his afternoon round with his longer putter. He needed only 26 putts, a day after he had 27.
When youre putting well, you dont really care if you hit it to 30 feet, you really dont, Bohn said. Youre like, I can make this. Then typically you dont it 30 feet, you start hitting it 12 to 15 feet, and then you start running them in.
Bohn did that at par-3 13th, when his 7-iron landed 4 feet away. He knocked in the putt for birdie, offsetting his only bogey a hole earlier, to take control of the leader board.