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 British Open champion 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.
U.S. Senior Open: Colin Montgomerie shot 6-under 65 to lead at the end of the first round at Oak Tree National.
The Scotsman started on No. 10 and birdied Nos. 14, 15 and 16 on the way to a 33 on the back nine. He birdied six, seven and eight to finish strong in oppressive heat and humidity.
Marco Dawson was second after a 66, and Mark Brooks was third after shooting 68. Bernhard Langer was one of five golfers tied for fourth with a 69.
Dawson, in his first Senior Open, started on the back nine and shot 2 under, then was steady on the front nine before scoring birdies on seven, eight and nine. The 50-year-old said it was one of the two best rounds he’s played in his six Champions Tour events.
Langer, who entered the Senior Open having already won three events this year, was in a five-way tie for fourth along with Vijay Singh, Kirk Triplett, Scott Dunlap and Gene Sauers.
Kenny Perry, last year’s winner, shot a 75.
Scottish Open: Rory McIlroy rolled in six birdies in seven holes around the turn and shot a 7-under 64 for a one-stroke lead midway through a low-scoring first round.
Refreshed after a two-week break, the No. 8-
ranked McIlroy tamed windy conditions in northern Scotland to show his links game is in good shape heading into next week’s British Open at Hoylake.
One of the highlights of McIlroy’s course-record round at Royal Aberdeen was driving the green on the 436-yard 13th hole while the group ahead was on the putting surface. That set up one of his eight birdies.
Unheralded Swede Kristoffer Broberg, out in the first group at 6:30 a.m. local time, and Ricardo Gonzalez of Argentina shot 65s. They were one shot behind McIlroy on a leaderboard that included Luke Donald at 4-under 67.
Phil Mickelson missed a 3-foot par putt at the final hole for his only bogey in a 68, a solid start to the defense of his title.
Ollie Schniederjans, the Cobb County native, Georgia Tech star and world’s No. 1 amateur, shot even-par 71.
Women’s British Open: Ayako Uehara of Japan got her one mistake out of the way early and opened with a 4-under 68 to lead the Women’s British Open.
Michelle Wie couldn’t stop making them.
Coming off her first major at the U.S. Women’s Open, Wie missed half of her fairways and stuck to a conservative plan on the par 5s that produced only one birdie. That was on her final hole at Royal Birkdale and allowed her to salvage a 75.
Even with only a mild breeze off the Irish Sea in the morning, Royal Birkdale was plenty difficult. Halfway through the opening round, only seven players were under par. Uehara and Mo Martin (69) were the only players to break 70.