Choi in charge at LPGA US Women's Open
Choi on Saturday had a 54-hole total of eight-under 208, with her nearest rival, compatriot Amy Yang, a distant second on two-under 214 after a three-under 69.
The two were the only players to break 70 in difficult windy conditions at Blackwolf Run.
Choi matched the lowest third-round score in tournament history and her 65 also tied the third-lowest round ever posted at a US Women's Open.
"I couldn't believe how I got eight birdies today," said Choi, ranked fifth in the world. "But I did. And I'm very happy, and I'm very satisfied and I'm very excited."
The lowest round in US Women's Open history was Helen Alfredsson's 63 in 1994. Three other players have shot a 64 in the Open.
Choi's impressive effort came on the same course where Se Ri Pak won the US Women's Open 14 years ago, inspiring a generation of South Korean girls to pursue golf careers.
Choi said she remembered watching that 1998 US Women's Open on television, and Pak's victory changed her ideas about what she herself might be able to achieve in the game.
"I changed my goal: 'I have to go to the LPGA Tour and I want to win on the LPGA Tour,'" Choi said.
American Lexi Thompson, Japan's Mika Miyazato and Germany's Sandra Gal were tied for third on one-under 215.
Thompson, 17, carded a 72, Miyazato a 73, and Gal signed for a 74.
It was a tough day for overnight leader Suzann Pettersen of Norway. She posted a 78 to move to one-over, but maintained she could still play her way back into contention.
"You know what, there's birdies out there," she said. "I think the wind is going to be a little bit less tomorrow from what I've seen.
"So if you get off to a hot start, hopefully put a number down early in the clubhouse. Who knows?"
American Michelle Wie, who had climbed up the leaderboard with a 66 on Friday to go into the third round one shot off Pettersen's lead, also posted a 78.
She, too, said a change in the weather could change everything on Sunday.
"I'm still not out of it," Wie said.
Yang, who had five birdies and two bogeys, said she'd try to stay steady on Sunday.
"I'm just going to keep being patient tomorrow, try to do my best," she said.
World number one Yani Tseng struggled to a 78 that left her eight-over par.
She said she had trouble with club selection in the face of the difficult winds and pin placements.
Tseng said she didn't see the course giving up many low scores "except for Na Yeon".
Choi started the day one-under par -- four shots off the lead.
The five-time LPGA Tour winner had four birdies on the front nine and birdied three in a row from the 10th.
The only blemish on her card was a three-putt bogey at 13, and she responded with a 15-foot birdie putt at 17.
"I have a good feeling about my putting speed and putting strokes," Choi said. "So I hope to get good results tomorrow."
But Choi acknowledged that it wouldn't be easy.
"Honestly, it will be a lot of pressure tomorrow," she said. "But you know, I know what I have to do, and I know what I can control."