Shane Lowry is joint-second in the RBC Canadian Open following a superb opening round of 64 at the Hamilton Country Club in Ontario.
Lowry joins a group which inlcudes Korean Im Sung-jae, Nick Taylor and Erik van Rooyen on six under par, while 2011 PGA champion Keegan Bradley birdied the 17th to climb to seven under par at the top of the leaderboard.
Lowry carded seven birdies in his opening round for the loss of just one stroke as he continued the good form that secured him a top-10 finish at the US PGA at Bethpage Black.
The Offaly golfer birdied holes 3 and 4 before dropping his only stroke of the day on the par-4 7th. He got back on track with a birdie on the 9th for an outward nine of 33 and hit a rich vein of form around the turn, birdieing 11, 12 and 14 to climb to five under. He rounded off an excellent round with a birdie on the 17th.
He held the clubhouse lead for a time until Keegan Bradley, who birdied six out of seven holes between the 7th and the 13th, rolled in his final birdie on the penultimate hole for a stunning opening 63.
Graeme McDowell, ahead of his return to the scene of his major triumph at Pebble Beach, is just one shot behind Lowry's group. He carded a five-under 65 in a round which included six birdies and one bogey.
Rory McIlroy carded a solid opening round of 67. The 2019 Players Champion registered five birdies and two bogeys to leave himself well in contention at this early stage.
Harrington, who narrowly missed out on US Open qualification earlier in the week, had a more mixed day, eventually finishing on one over par after a 71.
Waterford's Seamus Power suffered a mixed opening round, hitting two bogeys and two birdies to leave him on an even-par 70.
It was a similar day of frustration for Brooks Koepka, who also shot an even-par 70.
A week before going for a third straight U.S. Open title, there was nothing wrong with Koepka's long game, but he had trouble gauging the speed of the Hamilton Golf & Country Club greens, which he said were slower than he expected.
Heavy rain on Wednesday left the short 6,967-yard course soft and allowed players to fire aggressively at the pins, though it was still challenging due to rough that proved penal for errant shots.
Koepka, McIlroy and Johnson are among the big names playing in Hamilton with an eye towards peaking at Pebble Beach next week.
Despite his moderate score, Koepka, in his first start since winning the PGA Championship three weeks ago, was happy with everything except his putting -- and a wayward drive into a penalty area at the 10th.
"I liked the way I was hitting the putts, liked the way I was driving the ball, iron play was really good," the American told Golf Channel.
"I gave myself some good chances and didn't capitalise. Two birdies today, very easily could have been five or six, so disappointed with that but also at the same time pretty pleased."
Northern Irishman McIlroy described his tournament debut as "satisfactory" though like Koepka he felt he had left some shots out on the course.
"I really feel like it's the worst I could have shot," he said.
"A big improvement from how I played last week at Memorial. The work I did over the weekend, I definitely hit it much better today off the tee and that was a big key."