Zach Johnson among leading trio in Illinois

Johnson, the 2012 champion, carded an eight-under-par 63 at TPC Deere Run in Silvis, Illinois, matched by fellow American Harman and Sabbatini, who were also among the earlier tee-off times on Thursday.

Johnson and Sabbatini went bogey-free through their first 18 holes, while Harman carded nine birdies and a bogey.

So hot was Johnson's game, the world number 16 golfer said thoughts of breaking 60 were on his mind as he reached eight under with seven holes to play.

"It entered my mind very briefly. I don't want to say I should have shot 61 or 58 or 60 or 59 or whatever. But I had a chance," he said.

On a day of low scoring - just under two-thirds of the field shot even or under par - Americans Todd Hamilton and William McGirt produced rounds of 64, along with Australia's Steven Bowditch to share a tie for fourth.

Just two shots off the pace are American trio Robert Streb, David Toms and Kevin Tway after shooting 65s, and they are tied with Brendon de Jonge - after the Zimbabwean went bogey-bogey to finish his round on six under.

Defending a PGA Tour title for the first time in his fledgling career, Jordan Spieth finished the day with a 71 after battling his way to even par.

Spieth was two-over-par with three holes to play, but birdies on 16 and 17 gave the 20-year-old some momentum heading into Friday's second round.