Johnson, the 2012 champion, was joined by Rory Sabbatini and Brian Harmon atop the leaderboard at TPC Deere Run.
Todd Hamilton, Steve Bowditch and William McGirt shot matching 7-under 64s to sit in a tie for fourth place. Brendon de Jonge, Kevin Tway, David Toms and Robert Streb share seventh at 6-under 65.
Defending champion Jordan Spieth birdied two of his last three holes to finish his round at even-par 71, placing him in a tie for 81st.
Johnson was the lone member of the leaders to start on the back nine and opened his round with a birdie. After a par at the 11th, he rolled in a 19- foot birdie at the 12th to start a run of three straight gains to race to 4- under.
He stayed hot with back-to-back birdies from inside five feet at the 16th and 17th to move to minus-6.
Johnson converted another pair of birdies at the first and second to reach 8- under and parred his final seven holes to end there.
"(Shooting a 59) entered my mind very briefly," Johnson said. "I don't want to say I should have shot 61 or 58 or 60 or 59 or whatever. But I had a chance."
Sabbatini also posted a bogey-free round on Thursday. After following an opening par with three straight birdies from the second, he moved to 4-under with a 17-foot birdie putt at the seventh.
Another lengthy birdie putt at the 12th moved him to minus-5, and he raced into a share of the lead with a trio of gains at the 15th, 16th and 17th. He parred the last to end at 8-under.
"The course is very fair," Sabbatini said. "The greens are receptive, but you have to put the ball in the right positions out there. It's very fair. That's the best way to put it. You hit good shots you are going to get rewarded and if you hit bad shots you get penalized."
Harman recorded birdies at the third, fifth and seventh to make the turn at 3- under.
He then reeled off four straight birdies from the 11th before a 2-putt bogey at the 15th dropped him back to 6-under.
He quickly got that stroke back by sticking his tee shot at the par-3 16th to two feet and converting that birdie effort. He followed that with a 2-putt birdie at the par-5 17th to reach minus-8 and parred the last to end there.
"I finally got the putter going. I haven't been putting very well. I made a couple little adjustments and I made a couple adjustments off the tee box. Sometimes it doesn't take much," Harman said.
Bowditch and de Jonge had also reached 8-under late in their rounds, but Bowditch bogeyed the 18th to lose his share of the lead and de Jonge tripped to bogeys at the eighth and ninth, his final two holes, to fall two shots off the pace.
NOTES: Johnson has now recorded 21 straight rounds under par at this event. He has finished inside the top three in each of his last three starts here, including a playoff loss to Spieth last year ... Harman's caddie was forced to give up his duties Thursday due to food poisoning, so a local girls golf coach for Allamen Catholic High School in nearby Rock Island took over starting at the seventh hole ... Patrick Cantlay withdrew before his first round with a back injury.