The 15-year-old phenom took sole possession of the lead after three rounds thanks to an even-par 72 at the Vancouver Golf Club. After sharing the lead the day before, Ko now sits at 8-under 208 and holds a one-stroke advantage over four other golfers heading into Sunday's final round.
A victory for South Korean-born New Zealander would make her the youngest player to ever win on LPGA Tour.
"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."
Jessica Shepley of Oakville, Ont., the lone Canadian remaining in the field, shot a 2-under 70 and now sits at 1-over 217.
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 U.S. is the youngest LPGA Tour winner, taking the Navistar LPGA Classic last September at 16.
"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 Saturday. She got as low as 10-under with birdies on the par-3 first hole and par-4 sixth. But 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.
Despite the miscue on the 18th hole, Ko has been a picture of composure that belies her youth. Contrary to perception, Ko said, she does get angry at times, but tries not to show her emotions.
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," she said.
Chella Choi, tied for the lead with Ko after the second round lead, struggled in with a one-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, posted an eagle three on the par-5 10th hole as well as four birdies. The 27-year-old Toledo, Ohio native, is in contention to win this tournament 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.