Phil Mickelson moved into a three-shot lead on eight under after firing a superb second-round 65 at the USPGA.
The American, who shared the overnight lead on three-under 67, eagled his ninth hole as he made a dazzling start.
A double bogey followed, but the world number four added three birdies and one more bogey to lead Jerry Kelly (65).
Lee Westwood, Rory Sabbatini and Davis Love are third on four under but Tiger Woods just made the cut on four over after sinking a birdie at the last.
Mickelson said the nature of the Baltusrol course means players have to forget any bad shots and focus on the next hole.
"What I was most proud of was that when I did make a mistake, I let it go. Sometimes it's hard to do that.
"When I made the double bogey, I came back with birdies. When I bogeyed six, I let it go and birdied eight.
"I'm feeling very confident, but there's a lot of golf left," said the 2004 Masters champion, whose halfway total of 132 was just one off the championship record.
The 38-year-old Kelly moved into contention thanks to a flawless round featuring birdies at the first and last.
Former champion Love shot a 68, South African Sabbatini a 69 and Briton Westwood a 68 to keep Mickelson in their sights.
"I hit the ball real solid and made some putts," said Love, who won the championship in 1997.
"This golf course will bite you if you don't pay attention to what you are doing, you've got to keep playing one shot at a time."
Westwood, seeking to become Europe's first PGA winner since 1930, has hired a psychologist to help him.
"I just felt like I wasn't thinking as clearly as I could do," he said.
"My last round scoring average has been very poor this year and I don't feel I've playing any worse in the last round, only thinking a bit worse."
Defending champion Vijay Singh of Fiji dropped a stroke at his last hole, the par-three ninth, to card a 67 and finish one shot behind the third-placed group on three under.
Level with Singh are Australian Stuart Appleby (70), Briton Greg Owen (69), Sweden's Jesper Parnevik (69) and Japan's Shingo Katayama (66).
South African Retief Goosen, last year's US Open champion, was a further stroke back after returning a level-par 70.
Former European Ryder Cup captain Bernhard Langer was three under with three to play but finished with a 73 to slip back to level par, the same total as John Daly (69) and Ben Curtis (73).
Michael Campbell (68) is one over, one ahead of Sergio Garcia (70), while Jose Maria Olazabal (67) and Miguel Angel Jimenez (72) just made the cut on four over.
Among those to miss the cut were Open runner-up Colin Montgomerie (71), who despite a closing eagle could not recover from his initial 77, Darren Clarke (77) and Padraig Harrington (77).