Chez Reavie shot to the top of the leaderboard after a second-round 62 at the John Deere Classic in Illinois.
The American, who shot a first-round 66, had an eagle at the second hole which was immediately followed by two birdies.
Reavie sunk nine birdies in all but had two bogeys to finish on 14-under, two strokes ahead of Steve Marino.
"The putter's been great, and that's been the key," the 29-year-old said after totalling only 21 putts.
Reavie, whose only PGA Tour victory came at the 2008 Canadian Open, conceded that he began thinking about a 59 after recording his ninth birdie of the day at the par-three seventh, his 16th hole.
"That's when I thought, 'I make two more birdies, I shoot 59'. And then I hit it up there to about 10 feet on the eighth hole, hit a great putt and it lipped out," he said.
Marino remains second after a round of 66 that included an eagle on the second, a bogey on the fifth and four birdies to move to 12-under-par.
Jhonattan Vegas moved up to third after a 64 as he looks for his second Tour win.
The Venezuelan sunk seven birdies, including three in a row from the second hole, to finish on 10-under after improving on his first-round 68.
Kirk Triplett also improved on his first round when he carded a 65 to move into a five-way share for fourth.
The American putted eight birdies, including five in the first seven holes, as well as two bogeys to move to nine-under par.
Alongside him is Arjun Atwal, who carded a 66, including six birdies and a bogey along with Billy Mayfair, John Mallinger and David Mathis after rounds of 66, 65 and 65 respectively.
Overnight leader Kris Blanks had a poor day, with a round of 71 sending him down to a tie for ninth.
The American had a woeful start with a double-bogey at the first hole, while four birdies were offset by two more bogeys, and he is now six shots behind leader Reavie.
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