South Korea's Kim Sei-young fired a bogey-free seven-under par 64 to seize a one-stroke lead over compatriot Lee6 Jeong-eun after Friday's second round of the LPGA Marathon Classic.
Kim ran off four birdies in a row starting at the third hole, added birdies at 12 and 15 and closed with a birdie at the par-5 18th to finish 36 holes on 11-under 131 at Highland Meadows in Sylvania, Ohio.
"Just doing my best," Kim said. "I try my best, and then if I play good, just keep it going.
"I made a couple long putts. Greens are very pure, so if I read it really good it works."
World number 12 Kim seized a one-stroke lead over ninth-ranked Lee6, who fired a second consecutive 66, with Americans Lexi Thompson, Jennifer Kupcho and Stacy Lewis sharing third on 133.
Kim is chasing her ninth LPGA victory and second of the year, having won the Mediheal Championship in May. Kim has at least one win a year since she debuted as the 2015 LPGA Rookie of the Year.
"I'm very excited, but I try to calm down because I have two more rounds," Kim said. "I'll just let it come to me."
Kim will try to capture her first major title in two weeks at the Evian Championship in France, where she shared second last year.
Lee6, who won her first major title at last month's US Women's Open, birdied three in a row starting at the par-3 sixth and opened and closed the back nine with birdies but a bogey at the par-4 12th kept her from sharing the lead.
"I did pretty good," Lee6 said through a translator. "My putting started improving. I didn't make any three-putts so I'm pretty satisfied with it."
South Korean 19-year-old rookie Chun Youngin, who shared the 18-hole lead after an opening 64, shot 70 to stand on 134, sharing sixth with Spain's Carlota Ciganda and Clariss Guce of the Philippines.
Spain's Azahara Munoz and Germany's Caroline Masson were another stroke adrift after each fired a 70.
Thompson fired a bogey-free 67 to jump into contention.
"I feel good with where my game is at," she said. "Just firing at pins and focusing on one shot at a time.
"I played some great golf. Just going to try continue with that into the weekend and see where it takes me."