Ryu So-yeon celebrated her 28th birthday with a three-under-par 69 that lifted her into a three-way tie for the halfway lead at the Women's PGA Championship outside Chicago.
The former world No.1, back in form after a spell in the doldrums, joined South Korean compatriot Park Sung-hyun (72) and Canadian Brooke Henderson (71) at six-under 138 at Kemper Lakes in Kildeer, Illinois.
They headed Spaniard Carlota Ciganda (69) by one stroke, while New Zealander Lydia Ko matched the best score of the week (66) to vault to within two shots of the leaders.
Defending champion Danielle Kang (69) is in a group tied for 12th at two under.
Australia's contingent is led by Sarah Jane Smith (72) and world No.9 Minjee Lee (76) at one over while veteran Karrie Webb (76) is a shot further back.
A day after feeling that a three-under 69 did not reflect how well she had played, Ryu admitted an identical score on Friday was perhaps more than she deserved.
"Golf is definitely a funny game because yesterday I thought I hit it just really, really perfect, like I just couldn't ask more, and I shot three-under," she told reporters.
"And then compared to yesterday, I felt like I absolutely hit it everywhere out there but still shot three-under.
"That was really good learning (experience)."
Henderson, the 2016 champion of the major, made a 15-foot birdie putt at the last to earn a share of the lead.
"Up and down all day," she said of a round that included five birdies and four bogeys.
Meanwhile, South Africa's Lee-Anne Pace was disqualified for using a non-conforming club.
She damaged her sand wedge when slamming it against a stake but did not realise she had done so until she had used the same club again a few holes later.