Sei Young Kim birdied five of the final six holes at Aronimink to post a five-under 65 for the second-round lead in the Women's PGA Championship.
The 27-year-old South Korean closed with a six-under 29 on the front nine. She dropped early strokes on the 11th and 12th holes and made her lone first-nine birdie on the par-5 16th.
Kim's four-under 136 total on Friday tied Australians Karrie Webb (2001) and Sarah Kemp (2011) for the lowest nine-hole score in the Women's PGA Championship.
A 10-time LPGA Tour winner, Kim is looking for her first major victory. She was runner-up at the 2015 Women's PGA Championship and tied for second at the Evian Championship in 2018.
The last four winners of the tournament either led or co-led after 36 holes.
Jennifer Kupcho (65), Danielle Kang (69), Carlota Ciganda (69) and Anna Nordqvist (68) are a stroke back.
Defending champion Hannah Green recovered from her disastrous opening-round 79 to post an impressive four-under 66.
The Australian's solid round - only bettered by Kim, Kupcho and Bianca Pagdanganan - ensures she will be there for the weekend in joint 46th position.
Compatriot Minjee Lee posted a one-over 71 to be four-over overall, eight behind Kim.
Katherine Kirk posted 73 which was just enough to make the cut at six-over for the tournament.
Su Oh, Sarah Kemp and Saraj Jane Smith will all be going home, but Oh had the distinction of landing the first eagle of the tournament at the 16th when she holed a green-side bunker shot.
Kupcho was second last weekend in the ShopRite LPGA Classic and said a run at the championship left her exhausted.
The 23-year-old Kupcho hit all 18 greens in regulation.
Kupcho, winner of the inaugural Augusta National Women's Amateur, has yet to transfer the success of her celebrated collegiate career over to the LPGA Tour.
She was the No.1 ranked amateur at Wake Forest and won the 2018 NCAA Division I Golf Championship.
At majors, she failed to make the cut four times, including at the last two Women's British Opens.
At Aronimink, Kupcho went back to what worked for her in college. She decided to use the irons she played with as an amateur after a talk with her parents.
Kupcho played each nine at Aronimink only once leading into the tournament and found the unfamiliarity with the sloped greens may have worked to her advantage.
"Going into last week I was not prepared, like I felt completely unprepared," she said. "I felt the same way coming into this week.
"I think not being prepared brings another challenge for me and I think that's what helps me play well."
Nelly Korda, the No. 2-ranked player in the world, dropped out with a back injury after she shot a 71 in the opening round. ANA Inspiration winner Mirim Lee also withdrew because of an injury.