Marina Alex eagled her final hole for an eight-under 64 and the second-round lead on Friday at the LPGA Tour's Manulife Classic.
Alex played the final five holes in five-under for the lowest score in her LPGA Tour career, setting up the 18-foot eagle putt with a five-wood approach on the par-five ninth.
The 26-year-old American is at 10-under overall at Whistle Bear for a one-stroke lead over Thailand's PK Kongkraphan and South Korea's Hyo Joo Kim.
Minjee Lee carded a 69 to sit five shots off the pace at five-under while Sarah Jane Smith (72) at one-under was the only other Australian to make the cut.
Compatriots Karrie Webb, Su Oh, Katherine Kirk and Sarah Kemp won't be around over the weekend.
World No.1, New Zealand's Lydia Ko, was two-under after a 72.
"Super happy," Alex said.
"Just kind of everything fell together, ball-striking, putting. I actually left a couple of putts out there, but I'm not going to complain about the score that I shot."
Second-ranked Ariya Jutanugarn was four strokes back after a 68 in her bid to win three straight events for the second time this season.
Using a driver in competition for the second straight day after shelving it for months, the 20-year-old Thai player had an eagle, six birdies, two bogeys and a double bogey.
She began the streak at the Women's British Open, and rebounded from a knee injury that forced her to withdraw during the Rio Olympics to win the Canadian Pacific Women's Open on Sunday in Alberta.
US Open champion Brittany Lang, second last year at Whistle Bear after winning the inaugural event in 2012 at Grey Silo, was six-under after a 71.
First-round leader Chella Choi also was six-under after a 72. The South Korean player three-putted three times.
Defending champion Suzann Pettersen had a 71 to reach four-under.
Canadian star Brooke Henderson, playing alongside Jutanugarn, was three-under after a 71. She eagled the par-5 12th and closed with a birdie.
The 18-year-old Henderson beat Ko in a playoff in June in the KPMG Women's PGA for her first major title, and successfully defended her Portland Classic title in early July.
