The Korean-born teenager, four days after her silver medal- winning performance at the Rio Olympics, wasted no time getting back into the bread-and-butter swing of regular LPGA competition.
She completed her round in style by sinking a tricky five-foot par putt with her assured cross-handed putting stroke at the scenic Priddis Greens Golf & Country Club in Calgary.
Ko made the Canadian Women's Open her first LPGA victory when she triumphed in 2012 at the age of 15. She added further wins at the tournament in 2013 and again last year.
"The course and the environment really remind me of New Zealand," world number one Ko (19) told the Golf Channel after a round that included six birdies and one bogey.
She conceded it was a challenge to stay mentally sharp after the enjoyable but draining experience in Rio, where she finished five strokes behind golf medallist Park In-bee as golf returned to the Olympics for the first time in more than a century.
"The Olympics was surreal, a week like I've never experienced before," Ko said.
"Coming into a big week where I'm also defending, it is tough because you're coming in with so much adrenaline and because I've played well a little bit more added pressure, but I love being here."
Ko was one stroke behind English leader Stephanie Meadow with half the field back in the clubhouse.
