American Daniel Summerhays fired a nine-under-par 62 to grab a two-stroke lead over Canadian David Hearn heading into Sunday's final round of the John Deere Classic in Silvis, Illinois.

Defending champion Zach Johnson, the 2007 Masters winner and overnight co-leader, was lurking just off the pace another shot back at 16-under-par 197 after posting 67 at TPC Deere Run in the third round on Saturday.

Summerhays, looking for his first PGA Tour win, had 10 birdies against a lone bogey in ideal scoring conditions, using a string of three in a row from the 15th hole to vault to the top of the leaderboard.

"I think when I'm playing well the mentality is make as many birdies as you can," Summerhays told reporters.

"I'm really looking forward to (Sunday). I'm playing really well."

"I think when I'm playing well the mentality is make as many birdies as you can" - Daniel Summerhay

Hearn, also seeking his maiden tour victory, posted eight birdies and one bogey in his 64, running off three successive birdies over the same stretch of holes from 15.

Tied for fourth on 198 were Americans J.J. Henry (65) and Jerry Kelly (66).

Australian Matt Jones and Americans Nicholas Thompson and Chris Kirk were another shot back.

Two-time US Junior Amateur champion Jordan Spieth, 19, was among three players on 200, six shots off the pace after three rounds.

Summerhays blew a two-shot lead during the final round of last year's Mayakoba Golf Classic in Mexico but said he was more relaxed this time.

"I know there's going to be obstacles and challenges, as there always are," he said.

"There are always things that stand in your way. But I'm just excited to see what those are and deal with them."

Johnson sank not one but two massive putts on Saturday, a 60-foot eagle putt on the par-five second hole then a 42-foot birdie putt on the par-four 14th, to surge into 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."