"It's not something I'm familiar with," Reto said about the low score at Ottawa Hunt and Golf Club.
Winless on the LPGA Tour, the 32-year-old South African opened with a six-under 29 on the back nine and added three birdies on the front nine - the last on the par-5 ninth. She birdied all four par-5 holes.
"It was a good putting day," Reto said. "A lot of putts went in and that kind of makes the round easier. Seeing my lines and the speed was pretty good, so made it a lot easier. I was like, 'Just get on the green.'"
Reto broke the course mark of 63 set by Canadian star Brooke Henderson in the third round of the 2017 tournament.
"I didn't know it was the course record, so kind of finding out after my round takes the pressure off," Reto said. "So I'm happy about it. You know, I'm hoping to do some more good rounds coming here."
Narin An of South Korea was second after a bogey-free 64.
Emma Talley, Robynn Ree and Lilia Vu shot 65. Atthaya Thitikul was at 66 with Perrine Delacour, Peiyun Chien, Wei-Ling Hsu, Nasa Hataoka, Lucy Li, Mo Martin, Elizabeth Szokol and A Lim Kim.
Alena Sharp and Maddie Szeryk were the top Canadians at 67. Nelly and Jessica Korda also were at 67 in a group that included Jennifer Kupcho, Danielle Kang, 2006 winner Cristie Kerr and 2016 champion Ariya Jutanugarn.
Nelly Korda is coming off her first win of the year in the Aramco Series on the Ladies European Tour where she rallied from seven shots behind against sister Jessica.
"The greens are definitely really tricky," Nelly Korda said. "They're hard to read and kind of like in a sense a little too fast for the grass, where you feel like you're putting on glass."
Top-ranked Jin Young Ko, the winner at Magna outside Toronto in 2019 when the event was last played, shot a 68.
Joining her at three under are Australians Hannah Green and Sarah Kemp, while compatriot Karis Davidson is a further shot back.
Canadian amateur Lauren Zaretsky had a hole-in-one on the par-3 fifth in her 68. The 18-year-old is set to start college Monday at Texas Tech.
"My reaction was pretty calm," Zaretsky said. "I'm surprised. I haven't had a hole-in-one since 2016 and actually made a joke before this tournament that maybe I'll have a hole-in-one on live TV."
Three-time tournament champion Lydia Ko had a 69, playing alongside Henderson and Kupcho. Lexi Thompson shot a 70.
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