The 15-year-old phenom took sole possession of the lead after three rounds as she shot an even-par 72 at the Vancouver Golf Club. After sharing top spot on Friday, Ko now sits at 8-under 208 and holds a one-stroke advantage over four other golfers.
Jessica Shepley of Oakville, Ont., the lone Canadian remaining in the field, shot a 2-under 70 and now sits at 1-over 217.
Heading into Sunday's final round, Ko has a chance to make women's golf history. Ko, who was born in South Korea but has grown up in New Zealand, can become the youngest player ever to win this even and any other LPGA competition.
"It's good to stay at the top of the leaderboard," said Ko. "But my first goal was to make the cut and, hopefully, (finish) top-15 or something. But to be up there in an honour, especially playing against the world's best."
Ko can also become the fifth amateur to win an LPGA tournament and the first since JoAnne Carner took the Burdine's Invitation in 1969. No amateur has ever won the Canadian Open.
Lexi Thompson of the United States is the youngest LPGA Tour winner, having taken the Navistar LPGA Classic last September at 16.
Ko is already the second-youngest golfer to win a women's pro event on any tour, behind Brooke Henderson, 14, of Smiths Falls, Ont., who won a Canadian Women's Tour event in Quebec this year. Henderson, who missed the cut here, broke Ko's record by just two days.
But the modest New Zealander was surprised by her chance to make history in Canada.
"Yeah, 15-year-olds don't lead at an LPGA event all the time. ... I'm very surprised," she said. "But I've been playing really good golf, and I've been really confident with my game."
But unlike in the first two rounds, Ko struggled at times on Saturday. She got as low as 10-under with birdies on the par-3 first hole and par-4 sixth. However, she also bogeyed the seventh and ninth holes and then saw her four-foot par put lip out on 18 after she had stepped back from her initial approach to get a better look at her line.
"I'm definitely going to do some putting practice," she said.
Although she has a chance to make history in an LPGA event, Ko said she felt more pressure trying to live up to her status as the world's No. 1 amateur.
"The next thing is just playing against the big names," she said. "It's really hard to keep up with them."
But on Sunday, the big names will have to keep up with her. Expressing respect for her more seasoned opponents, Ko hoped she has not become too big of a target.
"It's quite nerve-racking, but I'm really here for experience and fun — and I'm having fun at the moment."
Chella Choi, tied for the lead with Ko after the second round lead, struggled in with a 1-over 73, dropping into the four-way tie for second with Stacy Lewis of the U.S., who fired a sizzling 6-under 66, and South Koreans Inbee Park (70) and Jiyai Shin (69).
Lewis, a two-time winner this year, made the most out of moving day — the nickname for the third round when golfers move up the leaderboard — as she posted an eagle three on the par-5 10th hole as well as four birdies.
"I finally got off to a good start, which I hadn't done any of the previous days," said Lewis.
The 27-year-old Ohio native is contending for this tournament's title for the second straight year. In 2011, she and Michelle Wie finished as runners-up to champion Brittany Lincicome.
Lewis also excelled at another LPGA tournament in Canada earlier this year. She tied for fifth at the Manulife Financial Classic in June. Now, it's just a question of whether she can surpass Ko.
"Suzann (Pettersen) and I were talking about it (Friday)," said Lewis. "This is our job, and we're working full-time on it. It's not supposed to be her job, and yet she's beating us. I think it's good for the game. She's obviously playing well."
Meanwhile, Shepley's self-esteem was also soaring as the 29-year-old posted her best score of the tournament. The low score came after she squeaked into the weekend with a 3-over total that was right on the cut line.
It's the first time since April she has played in the final two rounds of an LPGA event. With her parents and friends looking on, Shepley soaked up the home-country atmosphere.
"I was really relaxed and actually enjoyed playing golf today for the first-time in a long time," said Shepley, who is playing in her fifth Canadian Women's Open.
While Ko chases history and veterans pursue her, Shepley hopes to do well for Canada — and herself.
"I'd like to get into red numbers," she said. "My best finish out on the LPGA was a (tie for 27th), which was in Hawaii this year. So I'd like to have my best finish out here. That would be nice."