Rory McIlroy was left 10 shots off the lead at the Canadian Open after shooting a stuttering one-over par opening round on Thursday.
Playing his first tournament since a disappointing showing at the PGA Championship last month, McIlroy recovered from a slow start with three successive birdies around the turn to move under par, but bogeyed his last two holes to post 71.
Ireland’s Shane Lowry was five-under through his first eight holes but had to settle for a 64, three shots off the lead, after a less explosive back nine.
England’s Danny Willett and Scotland’s Robert MacIntyre were both a shot further back on five-under, leaving them well placed to remain in contention at the weekend.
Justin Rose went five-under at the 11th hole but made a double bogey at the 15th before finishing on two-under.
McIlroy was frustrated that only news of his driver being deemed non-conforming had become public when the same had happened to his rival, and eventual tournament winner, Scottie Scheffler.
Out early on Thursday, McIlroy continued to struggle with adjusting to his new driver, missing the fairway right on several occasions during his round.
Playing the back nine first at TPC Toronto, McIlroy started with two pars before a poor error from little more than a 100 yards out on the fairway at the 12th hole led to a bogey.
He would drop further off the pace at the tough 17th as a drive into the rough left him a 20-foot putt for par, which he failed to convert.
However, the Northern Irishman responded immediately by taking advantage of the course’s two par-five holes.
He got himself in strong positions off the tee to leave tap-in birdies at both, before making it three in a row by rolling in an 11-foot putt at the second.
The streak could, and perhaps should, have continued as McIlroy set up birdie chances at the next four holes, but failed to take any of them before finally holing a putt to stay one-under at the par-three seventh.
