Park's two-day total of 133 at the Grey Silo course put her one shot in front of Brittany Lang, Hee Kyung Seo and Shanshan Feng, who this month became the first golfer from China to win a major title when she captured the LPGA Championship.
Lang, in her seventh year on the LPGA Tour, has had 34 Top 10 finishes in her career but is still looking for her first victory.
Teenager Lexi Thompson was part of the group at 135 in a tie for seventh place, tied with Chella Choi, Katie Futcher, Karin Sjodin, Sun Young Yoo and So Yeon Ryu.
Also at 135 was first-round leader Sandra Changkija, a tour rookie whose 63 on Thursday was the best round of her brief career by eight shots. She settled for a 72 Friday.
Stacy Lewis, a two-time winner this year, also had a 64 Friday and was tied for 12th at 136.