K J Choi knew he was destined for a "special round" even before completing an outstanding six under par 64 to move into the lead at the midway point of the AT&T National in Pennsylvania.
Choi managed an opening 69, but was in inspired form as he matched the course record 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 on seven under.
Choi, who won The Players Championship earlier this year, said: "I didn't know, nor did anybody tell me [I was on target for the course record].
"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."
Choi's passage to the top was eased by struggles for overnight leader Adam Scott, who 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. Scott will resume at three under on Saturday.
Hunter Haas, co-leader with Scott overnight, shot a 74 to fall to level par.
Choi enjoyed a two shot lead over Chris Riley, Bo Van Pelt and Charlie Wi, who all went round in 66, while Justin Leonard (67) was also on five under.
Charles Howell, Bryce Molder, Jeff Overton and Kyle Stanley were one shot further back.