Troy Matteson went 10 under par to lead by three after the first round while Steve Stricker put himself firmly in contention to win his fourth straight John Deere Classic.
Matteson had 10 birdies split evenly over the front and back nines for a career best-equalling 61 to lead by three from Ricky Barnes.
Stricker, champion in each of the last three years, carded a six-under 65 to be part of a group of seven players tied for third, which also included South Korea's KJ Choi and England's Gary Christian.
Winning again would enable Stricker to fly to The Open next week having equalled the feat achieved by Young Tom Morris at Prestwick in 1968-72 and also by Walter Hagen at the USPGA Championship, Gene Sarazen in the Miami Open and Woods at both Bay Hill and the Buick Invitational.
He said: "I'm looking at it as an opportunity. It's fun. But there is pressure involved. You're trying to do it and there's a lot of expectations not only from me, but from a lot of other people to do it too.
"I try to tell myself, I've won it three times. How in the heck can I win to the fourth time?
"I'm trying to downplay it to myself. Maybe it's working. I've done that every year here. Every time I come back I'm like, oh, there is no chance I can win again."
Stricker started slowly but made a charge on the back nine.
Level for the front nine and two under through 13, he eagled the par-four 14th before picking up further shots at 15 and 17.
Matteson, meanwhile, had a shot at breaking 60 if he eagled his last hole, the ninth.
Instead he got in trouble for the first and only time in his round, finding a greenside bunker, but still rescued par.
"I made a couple of putts that were really, really difficult, and I had a bunch of other really, really good looks," he said.
"Any time you do that and your putting starts to come around you're going to make putts.
"I didn't think it was going to be 10-under, but definitely thought it was going to be good."
Barnes carded eight birdies and a bogey to sit second, while Choi had six birdies in his round and Christian posted seven birdies against one bogey.
At the end of round one, 99 of the 156-strong field were under par.
They did not include England's Brian Davis or Scotland's Russell Knox, who were both three over.