KJ Choi hit a second-round 64 to match the Aronimink course record and take a two-shot lead at the AT&T National.
The 41-year-old South Korean, who won the Players Championship in May, birdied five of his final six holes to finish the day on seven under.
Justin Leonard, Chris Riley, Bo Van Pelt and Charlie Wi are in the chasing pack on five under.
England's Justin Rose is two over after a 72 while Brian Davis is one shot further adrift.
Anthony Kim, Jim Furyk and Camilo Villegas all missed the cut.
Overnight leader Adam Scott could only manage a one-over-par 71, his day marred by a double bogey on the 14th and bogeys on the third and 17th. The Australian is on three under.
Choi's blistering round equalled the mark Rose set in the second round last year in Pennsylvania.
"I didn't know, nor did anybody tell me [I was on target for the course record]," said Choi.
"But as I kept making birdies, I noticed that the reactions of the gallery, the people were very positive. I started feeling that this was going to be a special round.
"I think my putter felt good starting from the first hole. I sort of had a trust within myself that I'd be able to play a better front nine [than the first round] and I was able to make five birdies. So I think that was the key."