Young-A Yang carded a 5-under-par 67 and is alone in second place, followed by Donna Andrews, who is alone in third after an opening-round 68.
Park ran home a pair of 10-foot birdie putts at one and three, then drained an 8-foot birdie putt at No. 5. At the next hole, Park drove left off the tee, then hit a 5-iron over the green. She could not get up-and-down for par, but tapped in a 3-footer for birdie at seven.
She birdied the 10th, but double bogeyed the 12th when she missed the green and three-putted. Park collected back-to-back birdies at 13 and 14, then hit a 3-iron over the green at the par-3 15th. She chipped to 20 feet and two-putted for a bogey.
Park was two behind Yang, but knocked a 7-iron to 9 feet to set up birdie. At the final two holes, Park played 4-irons and drained birdie putts from 8 and 6 feet, respectively.
'Today I didn't expect to be leading,' said Park. 'I don't want to think about it.'
Park came very close to withdrawing before her tee time on Friday. She hurt her wrist, thanks to playing 12 straight weeks, and came down with a stomach ailment. Park asked officials if there were any alternates available and there were none, so Park decided to give it a go.
'I didn't feel like playing, but I decided I would try,' admitted Park, a two-time winner on the LPGA Tour. 'Yesterday my stomach was hurting and I threw up and just had an awful day. My wrist is still hurting from last week.'
Yang started on the back nine and tallied four birdies on her opening nine. She missed a 10-foot par putt at the second, but recorded back-to-back birdies from the fifth.
She parred out for sole possession of second place.
'I started pretty hot and thought I have to keep it going,' said Yang. 'The course played easier for me this year. I think my experience cannot hurt. I feel I've gotten over the stumbling.'
Andrews flew out of the gate with five birdies on the front nine, but dropped a pair of shots in a four-hole span to start her second nine. She hit an 8-iron to 10 feet to set up birdie at the 16th and get within two of the lead.
Laura Diaz, Marilyn Lovander and Nadina Taylor are tied for fourth place at 3-under-par 69. Clarissa Childs, Tammie Green, Moira Dunn, Kristi Albers, Kate Golden, Emilee Klein, Isabelle Beisiegel and Patricia Baxter-Johnson share seventh at minus-2.
Defending champion Rachel Teske struggled to an opening-round, 3-over-par 75.