In a dynamic final round that included everything but a winner, Mickelson went from three shots ahead to two shots behind Steve Elkington in a span of seven holes, only to recover when others got swallowed up by deep rough and the pressure of trying to win the final major of the year.
No one could have imagined Mickelson would struggle after the way he started.
And despite a 39-minute delay just before the leaders teed off, no one could have guessed that Tiger Woods would have the best 72-hole score at the end of Sunday.
After bogeying two of the first three holes to seemingly lose any chance, Woods finished with birdies on the last two holes for a 2-under 68, putting him at 2-under 278, two shots behind.
But the final two holes are par 5s, making it unlikely that Mickelson, Elkington and Thomas Bjorn at 3-under, or Vijay Singh and Davis Love III at 2-under, all will drop shots down the stretch.
Even so, Woods had to stick around New Jersey just to make sure.
``I had a wonderful four tournaments,'' Woods said of his run through the majors. ``I won two, I was close in one, and I don't know about the other one yet.''
If Monday follows form, it could be quite a show.
``I'm starting to hit some good shots, and we've got some birdie holes coming in,'' Mickelson said.
Half of the 12 players who did not finish the final round still have a chance to win.
Mickelson had a three-shot lead when he rolled in an 8-foot birdie putt on the fourth hole, but he couldn't run away from the field this time. Lefty lost the lead by making four bogeys in a five-hole stretch, either hitting into the rough or into the bunkers, and losing his touch on the greens.
Elkington, on the 10-year anniversary of his PGA Championship victory at Riviera, made all pars through the first seven holes -- Retief Goosen was the only other player among the contenders not to drop a shot over that brutal stretch -- and took the lead by chipping in behind the 11th green. But he looked tentative missing 8-foot par putts on the 13th and 15th holes to fall to 3-under.
Bjorn got back into the mix with a 40-foot birdie putt on the 13th hole, and he was one shot behind with four to play.
Singh looked as frustrated as ever.
Trying to join Woods as the only back-to-back PGA champions in the stroke-play era, he two saw his ball horseshoe around the cup and out as he made double bogey on the third hole and three-putted down the steep ridge on the par-3 fourth to quickly lose ground. The 42-year-old Fijian made one birdie from 3 feet on No. 8, but saw a half-dozen other chances slide by the hole.
Still, he was only two shots behind with three holes left, the same spot he was in last year at Whistling Straits when he hung around long enough to get into a three-man playoff.
Somehow, Love, too, was still part of the equation.
He also dropped four shots in five holes early in his round, and to his surprise Mickelson and Elkington came back to him. He was in the last group with Mickelson, and will have four full holes to change his fortunes.
No one knows what awaits Monday morning.
Overnight rain could take some of the fire out of Baltusrol, allowing players to attack the flags. But it also will make it tough for Mickelson, Love and Singh to reach the par-5 17th at 650 yards.
``This is a tremendous advantage, I think,'' Mickelson said. ``We get a few extra holes to play, and hopefully calm weather after hopefully some rain will maybe soften it up a bit.''
The course was getting firm and fast, and it showed.
Only three players finished four rounds under par. Along with Woods, Geoff Ogilvy and U.S. Open champion Michael Campbell each shot 69 to post 279. Six others were under par and still on the course.
No one could have imagined such a tight leaderboard based on the opening five holes, where Mickelson must have felt like he was part of a ticker-tape parade.
No one could hear the announcer on the first tee because fans already were screaming out Mickelson's name. And the noise got louder when he built a three-shot lead with the birdie on the par-3 fourth. Mickelson ambled toward the fifth tee and couldn't help but reach out and tap fists with the fans.
And then they went silent.
Mickelson has never had such a large lead on the last day of a major -- he had to rally to beat Ernie Els at the Masters last year -- and he quickly let everyone back into the game. He made two bogeys from the rough, two more from the bunker, and suddenly found himself two shots behind Elkington.
The 42-year-old Aussie has virtually vanished since his last PGA Tour victory six years ago at Doral, and he made his move quietly, by making pars.
But he began to succumb to the deep rough and fast greens, and as quickly as he took the lead from Mickelson, he gave it right back.