Johnson and Sabbatini played bogey-free golf, while Harman had nine birdies and one bogey on the par-71 TPC Deere Run.
They led 2004 Open Championship winner Todd Hamilton, Australian Steven Bowditch and William McGirt, the best afternoon finisher, by a stroke. Brendon de Jonge, Kevin Tway, David Toms and Robert Streb are two back at 6-under 65.
Johnson, who won the Deere in 2012 and lost it to Jordan Spieth in a playoff last year, birdied four of his first five holes and was 6 under on his opening nine.
Spieth opened at even par 71, rallying from 2 over with three holes to play with birdies on the 16th and 17th.
Sabbatini scored his best round of the year, closing with birdies on three of the last four holes.
Harman, who bettered his best round of the year by two strokes, was 2 under`through six holes when his caddie, Scott Tway, took ill. Jay Hatch of Davenport, Iowa, a high school basketball coach, volunteered from the gallery and carried Harman's bag the last 12 holes, which Harman played in 6 under.
Harman didn't miss a beat when his caddie took ill.
"I called a medic over, and Scottie said he was going to have to sit out at least a couple holes," Harman said. "Jay was standing there and said, 'I'll do it. I'll keep up.'"
Checking his own yardages, Harman birdied seven of his last 12 holes and nine overall.
"If you go out and birdie half of the golf course, you feel you've done fairly well," Harman said.
Hamilton's 64 came in his first round of the season on the PGA Tour. Playing on a sponsor's exemption, the 48-year-old native of nearby Galesburg, Illinois, has been on the Web.com Tour in recent years.
Bowditch was 8 under after a birdie at the 17th but bogeyed the 18th. McGirt birdied the first three holes on the back nine to jump into contention and parred the last four.
Three-time Deere winner Steve Stricker fired a 68, with six birdies and three bogeys.
Johnson won the Tournament of Champions in January but described his recent golf as "going through the motions too much." He hasn't finished in the top 10 since a tie for sixth in the Texas Open in March.
His start Thursday was a return to form, an early birdie binge including a 22-footer on the par-3 12th and a 32-footer on the par-4 14th. At 8 under through 11 holes, the vision of a 59 crept into his thoughts.
"Very briefly," Johnson said. "If you're going to do it, you still have to hit it solid. Maybe I misclubbed on 6, but I could have birdied 8. I hit only one really bad shot, and that was the second shot at 9."
Johnson parred his last seven holes.
"I'd rather have 59 wins than shoot 59," Johnson said.
Sabbatini, whose best finish this season is a tie for eighth at the Wells Fargo Classic, closed strongly on the hilly Deere Run layout.
"I like this course and how it plays," Sabbatini said. "It's fun for spectators because there are a lot of fireworks, and fun for us players, too. This golf course requires the basics: hit the fairway, hit the green, make some putts."
Four of Sabbatini's eight birdies were from 17 feet or longer, the longest a 22-footer on No. 4. He ran in birdies on the second, third and fourth holes of his back nine, and another on his 16th, the par-3 seventh.
Hamilton, a decade removed from his Open Championship victory at Royal Troon, birdied his first two holes and was off on an improbable round of 64.
"I haven't been playing well," Hamilton said. "It seems if I play well, I shoot 71 or 70. Everyone else when they play well, they shoot 65 or 64. It's a big discrepancy."