An had her second straight bogey-free round at Ottawa Hunt and Golf Club after opening with a 64 on Thursday, giving the 26-year-old South Korean a 13-under 129 total.
"I do have some experience in Korea, so it's a not an unfamiliar feeling right now," An said. "I hope to put up good results the next two days just like I did yesterday and today."
Minutes before darkness forced the suspension of play in the round delayed for two hours in the morning because of heavy rain and lightning, Nelly Korda closed with a birdie on the par-5 ninth for a 63 and a tie for second.
The third-ranked Korda was 11 under with Hye-Jin Choi of South Korea and first-round leader Paula Reto of South Africa. Choi had two eagles in a 68, and Reto followed her course-record 62 with a 69.
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 back against sister Jessica.
Lizette Salas was 10 under after a 64, birdieing the par-5 18th just before play was stopped. Danielle Kang (66), Nasa Hataoka (67) and Sarah Schmelzel (64) were nine under.
It was a big moving day for Stephanie Kyriacou, who shot a blistering eight-under 63 to move to within five of An.
The 21-year-old Australian, who opened with a 71, enjoyed a flawless round with eight birdies to shoot the joint lowest round of the day.
Her consecutive birdies on Nos 6-8 and Nos 11-13 to go with Nos 3 and 18, elevated her to joint ninth alongside Hannah Green.
The 2019 US Women's PGA champion's five-under 66 followed an opening 68, and her round included six birdies and a bogey.
They are two strokes better than compatriot Karis Davidson (69, 67), who is in a tie for 22nd.
Canadian star Brooke Henderson, the 2018 winner at Wascana in Regina, Saskatchewan, was five under after a 68.
"I feel like I'm hitting the ball really well, so I'm excited for the weekend," Henderson said.
Three-time tournament champion Lydia Ko also was five under after a 68, playing alongside Henderson and Jennifer Kupcho. Kupcho was seven under after a 68.
Lexi Thompson shot a 68 to get to four under.
Top-ranked Jin Young Ko, the winner at Magna outside Toronto in 2019 when the event was last played, missed the cut with rounds of 68 and 73.
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