Daniel Summerhays shot 9-under-par 62 on Saturday and moved ahead by a pair of strokes entering the final round of the John Deere Classic.
Summerhays, who missed three straight cuts before tying for ninth at last week's Greenbrier Classic, is 19-under 194 through 54 holes at TPC Deere Run. The 29-year-old will attempt to secure his first career PGA Tour win on Sunday.
"I think when I'm playing well the mentality is make as many birdies as you can," he said. "I'm really looking forward to (Sunday). I'm playing really well."
David Hearn shot 64 and moved into second place at 17-under, while defending champion and overnight co-leader Zach Johnson dropped into third at minus-16 after a third-round 67.
J.J. Henry (65) and Jerry Kelly (66) share fourth at 15-under, one stroke ahead of Nicholas Thompson (64), Chris Kirk (66) and Matt Jones (68).
Nineteen-year-old Jordan Spieth (65), Kevin Sutherland (65) and Patrick Reed (70) share ninth at 13-under.
Johnson posted his 19th straight round in the 60s at this event, but it wasn't enough to hold onto the lead.
He appeared poised to stay in front early when he drained a 60-foot eagle putt at the par-5 second. But the 2007 Masters champion stumbled to a bogey at the sixth and, after a birdie at the ninth, dropped another shot at the par-4 11th to fall down the leaderboard.
He rebounded with back-to-back birdies from the 13th and added a tap-in birdie at the 17th to remain in contention.
"I certainly think there were a lot more positives out there than negatives," Johnson said. "My putter has been great. Even the ones I've missed have been great."
Hearn opened his third round with back-to-back birdies and turned the trick again from the par-4 fifth. He tripped to his only bogey of the day at the ninth, but quickly recovered with a birdie around the turn at No. 10.
After four straight pars, Hearn ran off a trio of consecutive birdies from the 15th, then parred the last to move within a pair of Summerhays, who had just wrapped up his career-best 62.
Summerhays opened with consecutive birdies to kick start his outward 31, which featured back-to-back gains after a bogey at the third, and another birdie at No. 9.
Around the turn, Summerhays made it three in a row with birdies at Nos. 10 and 11, then ran off a hat trick of birdies from the 15th to go in front. He parred the last to stay there.
"I know there's going to be obstacles and challenges, as there always are. There are always things that stand in your way. But I'm just excited to see what those are and deal with them," he said.
NOTES: Summerhays matched the lowest third-round score in tournament history ... His best finish on the PGA Tour this season is a tie for seventh at the Valero Texas Open ... Amateur Patrick Rodgers is tied for 12th at 12-under after a third-round 65. No amateur has won on the PGA Tour in 22 years ... Three-time tournament champion Steve Stricker is tied for 17th at 11-under ... Jeff Overton withdrew during the second round with a wrist injury.