Jacqui Concolino followed a 67 with a 68 and was alone in fourth, a shot back.
The pack at 6 under included the world's top-ranked pro, Park and top-ranked amateur, Lydia Ko. They were joined by Chie Arimura.
Park is the hottest commodity in the LPGA these days, with six wins already — including the first three major championships — this year. She had won her last three starts before finishing 14th last week.
"It was a tough day. I just grinded for pars," she said after finishing off a 69 in heat and high humidity at Highland Meadows Golf Club. "The last two birdies (on the par-5 17th and 18th holes) felt like heaven."
The 25-year-old South Korean also said she felt she was pleased with her putting. In other words, the rest of the pack might want to look out.
Ko also birdied her final two holes to polish off a 67. The 16-year-old South Korean-born New Zealander, is in prime position to make a run at her second victory on the LPGA Tour. She was the youngest player ever to win a tour event last year at the age of 15 at the Canadian Women's Open, shortly after winning the U.S. Women's Amateur.
She easily made the cut in suburban Toledo, her 22nd in 22 starts against professionals.
Arimura had a 67.
Defending champion So Yeon Ryu (69), rising American star Lexi Thompson (71) and Jodi Ewart Shadoff (68) were all at 5 under.
"I'm ready to go low tomorrow," Ryu said after a session working on her putting. "There's always a little bit of pressure on my shoulders, especially if I'm the defending champion. But this is just a regular tournament. I'll just keep my focus on my ball and not think about other people."
Spoken just like someone who's won before.
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