Catriona Matthew and Angela Stanford shared the lead at 8-under-par 63 after the first round of the Manulife Financial LPGA Classic on Thursday at Grey Silo Golf Course in Waterloo, Ontario.
Inbee Park, the top-ranked women's player in the world, was two strokes back along with Irene Cho, Belen Mozo, Hee Young Park and Meena Lee.
Matthew had nine birdies and one bogey in the opening round. She birdied five of her last six holes, making a 25-foot putt to close out one of the best scores of her professional career.
"I hit it close all day, which gives you a lot of chances, and then obviously I putted well," Matthew said. "I gave myself a lot of chances but took advantage of them."
Stanford finished a bogey-free round with three consecutive birdies and said the softer conditions suited her game.
"Today, I hit it really good," Stanford said. "So it was just one of those days that as a golfer you just love.
"It's better for me. My ball tends to release more than others, so it's nice to have the greens a little bit softer. But I still thought they were firmer than I thought they would be and I think they'll firm up. If we don't get any more rain, the course will be perfect by Sunday."
Park is bidding to become the first player to win four consecutive LPGA Tour events since Lorena Ochoa 2008. The tour record is five in a row by Nancy Lopez in 1978 and Annika Sorenstam in 2004-05.
"There's a lot of imbalance in this game and to keep this kind of level going for four weeks, five weeks, six in a row is a very tough thing to do," said Park, who has six wins this year. "We really have to be strong mentally, you have to be physically strong."
Ryann O'Toole, Jenny Shin, Amy Yang and Ji Young Oh were tied at 66, three shots off the lead. Defending champion Brittany Lang carded a 69.