Sung Hyun Park twice ran off four straight birdies on her way to a 10-under 62 round and a one-shot lead over qualifier Yealimi Noh in the Thornberry Creek LPGA Classic.
Park, coming off a victory in Arkansas that returned her to No.1 in the world, was at 17-under 127 at the end of the second round.
No Australians survived to face the weekend in Wisconsin, after both Robyn Choi (four-under) and Katherine Kirk (three-under) did not make the cut.
Noh finished off her round with a six-foot birdie putt on the par-five ninth for a 65.
Ariya Jutanugarn shot a 64 and was two shots out of the lead, and the Thai suggested that a course giving up so many low scores - the cut was at five-under 139 - might be what makes it difficult.
"Actually, I feel like it's really tough for me because I felt like every hole, everybody have the chance to make birdie," she said.
Park made so many she lost track.
Starting with the par-five 13th hole, she ran off four straight birdies. She closed out her round on the front nine with four straight birdies.
"I made four birdies in a row twice today. I honestly didn't know," Park said through a translator.
"When I was eight-under today, I thought I was seven-under."
Mina Harigae (63) and Tiffany Joh (66) were at 14-under 130, while Shanshan Feng of China had a 67 and was four shots behind.
The biggest surprise was Noh, mainly because of the schedule she keeps.
Coming off a superb summer of amateur golf last year - she won the Girls Junior PGA, the U.S. Junior Girls and the Canadian Women's Amateur in three consecutive weeks - Noh decided not to play at UCLA and turn pro, even though she had no status on any tour.
She has received two sponsor exemptions on the Symetra Tour. She played a Korean LPGA event. And she has been trying to Monday qualify for LPGA Tour events, without much success. She finally got through this week, and is making the most of it.
"It means so much," Noh said. "All the hard work, all the months of just like not knowing where I'm going to play and what I'm going to do, just finally playing here and playing well is really like a dream come true."
Those dreams did not include a weekend tee time in the final group with Park, the No. 1 player in women's golf.
"Definitely not. I did not think about that," Noh said.
"But no matter who I play with and how they're playing, anything, I just don't want to think about anything else. Just focus on my score and my game. I don't want to think too much."
She also had to keep her eyes off the game next to her.
Presley Cornelius is a 20-year-old amateur who was given a sponsor's exemption into the tournament as a member of the Oneida Nation. She had told the Oneida website at the start of the week that she was proud to represent the tribe and "hopefully, it will give me an inside look of what could be and how this works".
She opened with a 96, and Friday was even more difficult.
Cornelius had every score on her card from a three to a nine. She had two triple bogeys, two quadruple bogeys and a quintuple bogey nine on the par-four fourth hole. It added to a 105, putting her at 55-over 201 for the two days.