Nick Watney fired an eight-under-par 62 to set a new course record and take the joint lead going into the final round of the AT&T National in Pennsylvania.
A superb inward nine that included six birdies and an eagle put him move to nine under with Rickie Fowler.
"I guess anything is possible," said Watney. "But I don't think you ever expect to shoot that low.
"You don't go on to the golf course very often anticipating a 62."
KJ Choi equalled the then course record of 64 on Friday to take the halfway lead.
But on a low scoring third round on Saturday, Chris Kirk bettered the record by one, Steve Marino also shot a 63 and then, less than an hour later, Watney set the new mark.
Fowler ensured co-leader status by shooting six birdies in his opening 10 holes on his way to a 64.
Overnight leader Choi birdied two of his last three holes on his way to a 69 that puts him one shot behind the front-runners.
Marino, Adam Scott (66) and Webb Simpson (64) are on seven under.
England's Justin Rose shot a 65 to get to three under, six shots off the lead, while Fiji's Vijay Singh withdrew from the tournament with a bad back.