Rory McIlroy made a strong start to the defence of his title at the RBC Canadian Open and he sits in joint fifth place at St. George's Golf and County Club.
Wyndham Clark fired a bogey-free, 7-under-par 63 to take the first-round lead.
It was three years to the day since McIlroy lifted the trophy, with the pandemic cancelling the last two years of the championship.
In 2019, McIlroy carded a stunning final round of 61 at Hamilton Golf and Country Club to seal a seven-shot triumph.
This time out he finished with a round of 66 tied with Tony Finau, Lee Hodges and Mackenzie Hughes.
"I think this course's best defense in any conditions is the rough and the pin positions," McIlroy said. "The greens are slopey. If you hit it in the rough, you don't have a chance of getting to some of the hole locations."
Shane Lowry was a shot further behind McIlroy after his round of 67 which started with a bogey but feature five birdies and left him in the clubhouse at -3.
Leader Clark made five of his seven birdies in an eight-hole stretch from hole 10-17 which was enough for a one-shot edge over England's Matt Fitzpatrick.
Clark - No. 293 in the Official World Golf Ranking - has never won on the PGA Tour but opened last week's Memorial Tournament with a 4-under round, one off the lead.
"Friday my nerves and some anxiousness out there kind of affected how I played, and I was really frustrated because my game was there last week," Clark said.
"I really felt like I could have turned that into a really great week, and I just didn't fully have it on the weekend. I played great Saturday and Sunday and shot like even-par both days. So it was just nice to come out here, felt similar play, but I just had the putts fall."
