Rory McIlroy shot a three-under-par 67 in his Canadian Open debut as Keegan Bradley took the early first-round lead in Hamilton, Ontario on Thursday.
But he was outshone by Offaly's Shane Lowry, who continued his good recent form, with a six-under-par at Hamilton Golf & Country Club.
Lowry fired seven birdies with just one bogey on the seventh hole to finish one behind Bradley.
McIlroy mixed five birdies with two bogeys. The Northern Irishman was the hottest player in the world for the first three months of the year, including victory at the prestigious Players Championship.
But he has cooled off lately, not challenging for victory at either of the first two majors of the season, the Masters and PGA Championship. He also missed the cut at the Memorial tournament last week.
Defending champion Dustin Johnson got off to a terrible start, though it could have been worse.
Johnson plunged to three over par after nine holes but recovered for a one-over 71.
World number 37 Bradley notched seven birdies to set the pace on seven-under 63, one shot ahead of Lowry, South Korean Im Sung-jae, Canadian Nick Taylor and South African Erik van Rooyen.
Johnson, McIlroy and other top players including world number one Brooks Koepka are competing in Hamilton with an eye towards sharpening their games ahead of next week's U.S. Open at Pebble Beach.
The Hamilton course was damp after torrential rain on tournament eve.
"I thought it was gong to be sopping wet but it dried out nice," American Webb Simpson told PGA Tour Radio after shooting 66.
"It's old style, doglegs, tough par-threes, gives you birdie opportunities throughout the day but still challenging."
If you haven't already, be sure to like our Irish Mirror Sport and Irish Mirror GAA pages on Facebook and follow us on Twitter.
