South Korea's Ryu So-Yeon stretched her lead to five strokes after firing a six-under par 66 in the second round of the LPGA Canadian Women's Open.
Ninth-ranked Ryu, the 2011 US Women's Open champion whose only other LPGA title came at the 2012 Toledo Classic, opened by matching the London Hunt and Country Club course record with a 63 and followed up with another solid effort to stand on 15-under 129.
"After I shot nine-under, six-under is not that great, but still it's a really great score and I made a lot of great putts," Ryu said on Friday.
"I had a bogey-free round two days in a row, so I feel really great about that.
"That's my goal. I aim for a bogey-free round all four days and I'm aiming for a really good win."
South Korean Choi Na-Yeon, American Danielle Kang and Sweden's Anna Nordqvist shared second on 134 with American Cristie Kerr another stroke adrift.
Ryu, who had 10 top-10 finishes without a victory last year and has nine more this year without lifting a trophy, birdied three of the first four holes to pull away after carrying a one-shot lead into the round.
She had another run of three birdies in four holes starting at the par-4 12th but said that she was prouder of going without a bogey than making 15 birdies.
"A lot of birdies still this time, but no bogeys is still more a great sign because it means I've played really consistent," Ryu said.
"When I was in trouble, I proved I can prove it with a bogey-free round, so I'm more happy with a bogey-free round."
Nordqvist fired a 69 but fell back with her first bogey of the event at the par-5 16th as playing partner Ryu seized command.
"I was trying to keep up with her on the greens. I played really well too. Just couldn't seem to make any putts. I burned six or seven edges so it was a bit frustrating, but she played really well and I tried to feed off her."
Choi had a bogey at the fifth and a birdie at the par-5 seventh, then began the back nine birdie-bogey before back-to-back birdies at 15 and 16 gave her a 70 for the round.
Kang concluded the front and back nines with birdies in a round of 66.
Two-time defending champion Lydia Ko of New Zealand fired a 69 to stand on 139, 10 strokes off the pace. The Kiwi teen star was an amateur when she took the past two trophies but is now in her rookie season on the LPGA circuit.