Pennsylvania - Britain's Justin Rose fired six birdies and had the day's lowest score to earn a one-stroke lead following the second round at the AT&T National on Friday while Tiger Woods barely made the cut.
Rose, who squandered a three-shot lead in the final round of last week's Travelers Championship, took advantage of benign afternoon conditions for a bogey-free six-under-par 64.
"I didn't really go out with the intention of going low. The round just evolved and happened," Rose told reporters after signing for a seven-under 133 halfway total at Aronimink.
Just one shot back are Australia's Jason Day (68) and South Korea's Charlie Wi (65), while another 10 players will head into the weekend within four strokes of the outright lead.
Defending champion Tiger Woods, who made the cut with no shots to spare, offset four bogeys with four birdies for an even-par 70 that left him 10 shots off the pace.
Woods drove the ball nicely but missed a couple of short putts, including one from two feet at his penultimate hole.
"It is frustrating," the world number one said. "The first two days is the best I've driven it in a long time."
While Rose, who won the Memorial tournament in Ohio last month, hopes to hang onto his lead this weekend, he is also hoping to secure a late exemption into the British Open this month at St Andrews.
He is almost certain to qualify this year, regardless of where he places this weekend, since he made the top two on a mini money list based on the last five tournaments on tour.
Rose said not making the British Open in 2000 and again in 2005 when he was the first alternate, is not an experience he wants to go through again.
"You see all your buddies teeing it up and going out there and experiencing the Old Course (at St. Andrews) in an Open, and I was left on the driving range as a spare part. I never wanted to do that again."
Day, meanwhile, sprayed his tee shots into the rough more often than not, but a deft short game kept him in contention.
"I hit it pretty awful on the back nine," said the 22-year-old who won the Byron Nelson Championship in May. "It's going to be tough if I have to do this for the next two days. I'm going to work on a few things on the range this afternoon and try to straighten it out."
Wi was much steadier in a bogey-free round that included three birdies and an eagle with an eight iron from 166 yards at the par-four 12th.