The 2008 US Open champion notched six birdies and just a lone bogey to move to seven-under for the tournament at Mission Hills Country Club in Rancho Mirage California, one clear of American Lizette Salas.
"I'm very happy with my play today on a tough golf course I think I handled myself very well especially with the wind," Park said.
"The back nine was pretty tough but I hit the ball where I wanted to and I putted very well today so that's why I got a good score today.
"I'm really excited that I could play for a win this weekend," she added. "That's just something really special but I'm not trying to think too far ahead, there are two more rounds so hopefully I can do my best and it's going to be enough."
Sweden's Caroline Hedwall (68) and Italian Giulia Sergas (69) share third at five-under.
Salas enjoyed plenty of local support and managed five birdies to one bogey as she stayed on the heels of Park.
Salas has three top-10 finishes this season and is hoping to ride her form to a maiden Major championship.
"I'm very pleased. I've been pleased with my ball striking all week," she said.
"I missed a few short putts on the front nine and I knew the wind was going to kick in on my back nine so I just really tried to stay patient and focus on dialling in those irons.
"I'm looking forward to more birdies. It's nice to be in contention at a Major but I can only take it one shot at a time."
Overnight leaders Choi Na-yeon and Suzann Pettersen both struggled to three-over 75s to drop back to one-under and a tie for 21st while world number one Stacy Lewis carded a 71 to be even-par and tied for 28th seven shots off the pace.
"I did a lot of good things but the score just didn't really show it," Lewis said. "I'm only a few back going into the weekend and on this golf course I'm right in it."
- Sports & Recreation
- Mission Hills Country Club