Sung Hyun Park charged to the top of the standings as overnight leader, while countrywoman Mirim Lee faded on day two of the U.S. Women's Open.
South Korea's Park was almost flawless, carding a second-round 66 to earn a two-shot lead at eight under through 36 holes on Friday.
Lee teed off with a three-stroke cushion but that quickly evaporated, following a two-over-par 74 which included a double-bogey and four bogeys, as Park posted seven birdies, only dropping one shot at CordeValle Golf Club.
"This is the first time in a USGA tournament. And coming to the tournament, I didn't even think about winning because this is the first time for me," Park said.
"I would like more experience with the USGA, LPGA. But I'm trying to enjoy this tournament. That's why I am just more comfortable; don't even think about the winning, I just enjoy the play. That's why it happened today."
Lee is tied for second alongside compatriot Amy Yang, who hit a 71 at the LPGA Tour event in San Martin, California.
World number one and New Zealand's teen sensation Lydia Ko made her move, holing a six-under-par 66 to be within three strokes of Park.
Haru Nomura (69) is also five under heading into Saturday's penultimate round.
Danielle Kang (69), Jessica Korda (70), Ji Eun-hee (71) and Kelly Tan (72) are tied for sixth, while Maude-Aimee Leblanc (69), Angela Stanford (70), Mo Martin (70), Jodi Ewart Shadoff (71) and Sydnee Michaels (72) round out the top 10.