PITTSFORD, June 9, 2013 (AFP) - Pressel had a 36-hole total of six-under 138 at soggy Locust Hill Country Club.
Park climbed up the leaderboard with a four-under 68 for 140, while overnight leader Choi carded a one-over 73.
The leaders face a demanding day on Sunday, with both the third and fourth rounds scheduled after rain washed out play on Thursday in the second women's major of the year.
Rain has continued over the last two days, leaving the course soft and in many places muddy.
Wet weather didn't faze Pressel, who birdied three of her first six holes.
"I got off to a good start, had a birdie opportunity on 10, but didn't go in and then made one on 11," she said.
"I started to hit my driver a little bit to the right, but made great par saves on 12 and 13, and then two great birdies on 14 and 15 to get me off to a really good and comfortable start."
After bogeys at 16 and 18, Pressel played a steady second nine that included a birdie at the par-five eighth.
She said she was gearing up for a tough test on Sunday.
"The question will be just how committed I can be to every shot because when you get tired your mind starts to wander," she said.
Park, whose Kraft Nabisco victory in April was her second major title after the 2008 US Women's Open, had five birdies and just one bogey.
"I hit the ball great today," she said. "I only missed, like, two fairways and three greens and it wasn't really in the long stuff today. So that was a big help.
"I made some mistakes yesterday, and I didn't do that again today, so that was good."
She opened her round with a tap-in birdie at the 10th. She picked up another shot at 12, and after her only bogey of the day at 14 -- where she three-putted from just off the green -- Park put together back-to-back birdies at 18 and one before grabbing her last birdie of the day at the par-five fourth.
"We still have a lot more golf to play," she said. "It feels like just one more round to go, but we have to play two rounds in one day."
She recalled the same situation at the Women's British Open last year.
"I think it's very tough physically, especially walking on these wet fairways, it's really hard on your feet. It's going to be tough, but I'm just going to try to concentrate every hole, just try not to lose my focus."
World number nine Shin Jiyai of South Korea headed a group sharing fourth place on 141, after a one-over 73. She was joined by compatriot Amy Yang, who carded a 70, and Australian Sarah Jane Smith, who posted a 69.
World number three Choi Na-Yeon carded a two-under 70 and headed a group on two-under 142.
Defending champion Feng Shanshan, whose victory last year made her China's first major golf champion, was tied for seventh at even par after a second-round 70.
New Zealand amateur Lydia Ko, already the youngest tournament winner in LPGA history, made the cut, bouncing back from a first-round 77 with a two-under 70 that left her tied for 40th.
Leading second-round scores here on Saturday in the LPGA Championship (USA unless noted, par-72):
138 - Morgan Pressel 68-70
140 - Park In-Bee (KOR) 72-68, Chella Choi (KOR) 67-73
141 - Sarah Jane Smith (AUS) 72-69, Amy Yang (KOR) 71-70, Shin Jiyai (KOR) 68-73
142 - Yoo Sun-Young (KOR) 73-69, Choi Na-Yeon (KOR) 72-70, Catriona Matthew (SCO) 71-71, Angela Stanford 71-71, Brittany Lincicome 69-73
143 - Caroline Masson (GER) 74-69, Chie Arimura (JPN) 71-72
144 - Michelle Wie 76-68, Feng Shanshan (CHN) 74-70, Pernilla Lindberg (SWE) 73-71, Danah Bordner 73-71, Ji Eun-Hee (KOR) 72-72, Laura Davies (ENG) 71-73, Lexi Thompson 71-73, Jessica Korda 70-74, Pak Se-Ri (KOR) 70-74
145 - Beatriz Recari (ESP) 74-71, Vicky Hurst 73-72, Kristy McPherson 73-72, Nicole Castrale 73-72, Suzann Pettersen (NOR) 72-73, Anna Nordqvist (SWE) 71-74, Hur Mi-Jung (KOR) 71-74, Lee Il-Hee (KOR) 71-74