A stroke ahead Thursday after an opening eight-under 62, the four-foot-11 Yamashita shot 64 in the afternoon to get to 14 under at Royal Mayfair.
"I'm really happy," Yamashita said through a translator. "With two rounds to go, I just want to focus on my game and keep playing like this."
The 25-year-old Japanese player won the Meijer LPGA Classic in June after taking the Women's British Open and Maybank Championship last year.
"Rather than focus on the score, I'm thinking about how I can make birdies," the 2025 LPGA rookie of the year said.
American Kiara Romero was tied for second in her first event as an LPGA Tour member. She followed an opening 64 with a 67.
The 20-year-old Romero is the second player to earn LPGA Tour membership through the LPGA Elite Amateur Pathway, following Lottie Woad last year.
"Honestly, I'm just really grateful to be out here," Romero said. "I just came out here with the mindset to just go out and have fun and just enjoy it and just remember that it's just golf at the end of the day."
Soo Bin Joo (63) and Yuri Yoshida (66) joined Romero at nine under.
"I like this golf course. I love Canada. It's so pretty," Joo said. "All the trees are so pretty and I'm enjoying it, except for the mosquitoes. Gosh, it's so bad."
Ruoning Yin closed with a double bogey for a 67, leaving her eight under with Nasa Hataoka (69).
Top-ranked Nelly Korda shot 70 to remain three under. Lydia Ko, the 2013 winner at Royal Mayfair as an amateur, was one under after a 69.
Karis Davidson (69-67) and major winner Grace Kim (67-69) are the leading Australians at four under.
They are tied 21st, one shot ahead of compatriots Robyn Choi (71-66) and Cassie Porter (69-68).
This is the final week of qualifying for Americans to make the Solheim Cup team for the September 11-13 matches in the Netherlands.
