Jordan Spieth charged into the lead with a stunning finish to his third round at the weather-affected John Deere Classic on Saturday.
The defending Masters and US Open champion played the final three holes in four-under as the shot a career-low 61 to move two strokes clear at TPC Deere Run.
Spieth was eight shots off the pace after his opening round but is now a strong position to claim a fourth victory of the year, at the venue where he claimed his maiden PGA Tour title two years ago.
After following up a tap-in eagle at the second with three more birdies in a front nine 30, the world No 2 put further pressure on the leaders with gains at the 13th and 16th.
The 21-year-old moved top of the leaderboard by holing out with a wedge from 107 yards at the par-five 17th, before extending his advantage with a 15-foot birdie at the last.
"Just hanging in there has been the key to this week." Spieth sad. "The one thing I regret is starting even-par after the first day and starting that far behind.
"We have made up for it, obviously, and still, the way this golf course is playing, I have to shoot well into the 60s to win this tournament tomorrow."
Last week's Greenbrier Classic champion Danny Lee is Spieth's nearest challenger, staying in contention by opening with a hat-trick of birdies on his way to a blemish-free 62.
Overnight leader Justin Thomas had the opportunity to join Lee in a tie for second at the 18th, but dragged a seven-foot putt for a second successive gain right of the cup to close a third round 69.
Thomas has company on 14-under in the form of playing partner Johnson Wagner and compatriot Shawn Stefani, who surged up the leaderboard with a birdie-filled seven-under 64.
Daniel Summerhays briefly held the outright lead after four successive gains from the seventh, but opened his back nine with back-to-back bogeys on his way to a third round 67.
Joining Summerhays in the group four back includes Zach Johnson, who was frustrated not to make more inroads over the closing stretch having picked up three strokes in his first three holes.
Watch the final round of the John Deere Classic live on Sunday from 6pm on Sky Sports 5.