Yamahita rebounded from a bogey on the par-3 17th with a birdie on the par-4 18th to get to 19-under 191 at Royal Mayfair, the tree-lined layout — and mosquito hot spot after record summer rain - tucked in a bend in the North Saskatchewan River.
The 25-year-old - one of professional golf's shortest players at 491cm (4-foot-11) - won the Meijer LPGA Classic in June in Michigan after taking the Women's British Open and Maybank Championship last season as the rookie of the year.
A stroke ahead Thursday after an opening with a course-record 62, Yamashita had a 64 on Friday afternoon to open a five-shot lead.
"I like this course," Yamashita said through a translator. "My feeling so good."
American Gina Kim (64) and Japan's Yuri Yoshida (66) were tied for second at 13 under, a shot ahead of Jennifer Kupcho (64) and two ahead of Somi Lee (65).
Aphrodite Deng, the 16-year-old amateur who won the 2025 US. Girls' Junior, was the top Canadian, shooting 63 to get to 10 under and be tied for sixth.
"I wasn't really counting like how low I was shooting," Deng said. "I was just trying to play as best as I can."
Cassie Porter (69) and Karis Davidson (70) were the leading Australians, tied 30th at four under, with Robyn Choi (70) and Grace Kim (71) a further shot back..
Canadian star Brooke Henderson, the 2018 and 2025 champion, was seven under after a 68.
"I hit 16 greens and gave myself a lot of good looks, so it's disappointing to only shoot 2 under," said Henderson.
Top-ranked Nelly Korda shot a 69 to get to four under. Lydia Ko, the 2013 winner at Royal Mayfair as an amateur, was 2 under after a 69.
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