First-round leader Scott slips back after a one-over 71
KJ Choi fired a best-of-the-day six-under-par 64 to move two shots clear at the halfway stage of the AT&T National in Pennsylvania.
The Korean was in inspired form as he matched the course record at Aronimink, with four straight birdies on the opening nine setting him on his way.
He made the turn in 30 and, although he bogeyed the 11th, back-to-back birdies on the 15th and 16th ensured he finished his round in top spot.
"I didn't know, nor did anybody tell me [I was on target for the course record]," he said.
"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."
He added: "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."
Chasing pack
Choi opened with a 69 but now leads by two from a group of four players that includes his fellow Korean Charlie Wi, who had a second round 66.
Former Ryder Cup player Chris Riley and his fellow American Bo Van Pelt both matched Wi's 66 to be joint second, while former Open champion Justin Leonard is also on five-under after a 67.
But overnight leader Adam Scott - who looked in superb form after firing an opening 66 - could only manage a one-over par 71, his afternoon marred by a double bogey on the 14th and bogeys on the third and 17th.
Defending champion Justin Rose finished just one shot inside the cut line of three-over after adding a 72 to his opening 70.
