World No1 Jason Day's spot on top continues to be threatened by world No2 Dustin Johnson after the American upstaged the opening of the Australian's title defence at the Canadian Open.
Johnson, the recent US Open and WGC Bridgestone Invitational winner, eagled the final hole to join the lead with a six-under-66 at Glen Abbey Golf Club.
Day earlier posted a three-under-69 to remain well and truly in the mix despite battling inaccuracy off the tee.
While Johnson cannot overtake Day at the top of the rankings this week, even with a victory, he can a week later at the PGA Championship and obviously the closer he gets, the more likely it is.
American Luke List joins Johnson with a 66 putting them one clear of fellow Americans Chesson Hadley and Kelly Kraft plus local Canadian amateur Jared du Toit and Spanish youngster Jon Rahm at five-under.
In dry and hot conditions Day, like others, found it tough to keep the ball in the fairways off the tee, hitting just four of 14 as his long ball rolled a mile.
He had similar struggles in his last few starts but managed to grind out the good score regardless, highlighted by a chip-in eagle on his 11th hole.
"It's definitely a premium on hitting a lot of greens this week but it's more so if you can hit the fairways, that gives you the opportunity to hit the greens," Day said.
"Had to change the game plan a little bit as it's just a little tough with how dry everything is, it's almost better to get up there around the greens with a wedge in your hand.
Day was hopeful an expected overnight thunderstorm might soften things up for his second round at the place he kick-started the best 12 months of his career but any such storm would also give Johnson a chance to put an early morning gap on the chasers.
Recent winner Greg Chalmers joined Day at three-under after three birdies in his final five holes. Chalmers is looking to make a run towards the tour playoffs to avoid being just the fourth player in history to have a win, but not make the post season.
Cameron Percy, Stuart Appleby and Rod Pampling are the next best Australians at one-under.
While most of the Aussies were solid Cameron Smith had a day to forget and Steven Bowditch's horror year continued. Smith shot a six-over 78, punctuated by a double-bogey six at the par-four 14th hole, while Bowditch is five-over after the opening round.