Park is seeking her second major title after winning the 2008 U.S. Women's Open. She picked up her fourth LPGA Tour victory in unexpected fashion in February in the LPGA Thailand when 17-year-old Ariya Jutanugarn closed with a triple bogey to blow a two-stroke lead.
"I just like to stay a little quiet and just do my own thing," Park said. "If I win a little more, I think I'll get a little bit more attention."
Park hit a sand wedge from 70 yards to 2 feet for birdie on the par-5 11th to take the outright lead, then hit 9-iron approaches to 7 feet on the next two holes to set up birdies. The wind pushed her tee shot on the par-3 14th into the left bunker, leading to her lone bogey, and she avoided further trouble on the final four holes.
"We were playing about a club and a half of a breeze there and it was getting tough," Park said. "I'm just happy that I have a chance on the weekend that I can win."
Sweden's Carolina Hedwall and Italy's Giulia Sergas were tied for third at 5 under. Hedwall shot 68, and Sergas had a 69.
Michelle Wie followed her opening 72 with a 70 to reach 2 under. She had six birdies, but three-putted the 18th for the last of her four bogeys.
"I made a lot of great birdies," Wie said. "The whole round was pretty interesting in general. I hit some interesting shots around the trees."
Playing partner Lydia Ko, the 15-year-old New Zealand amateur who won the Canadian Women's Open last August to become the youngest LPGA Tour champion, was 2 over after a 74.
"She is amazing," Wie said. "I think she's pretty good and, like I said yesterday, she is also a really good girl."
Top-ranked Stacy Lewis was seven strokes back at even par after a 71.
"It's definitely frustrating," said Lewis, the 2011 champion who won consecutive events this year in Singapore and Phoenix. "It's a major and you want to play well here, but that's golf. Some weeks, it's going to all come together and other weeks, it's going to be tough, and you've got to fight through it."