Jennifer Kupcho torched the Dinah Shore course at Mission Hills on Saturday, firing an eight-under par 64 to take a six-shot lead in the Chevron Championship.
Kupcho, a former amateur standout trying to make her first LPGA victory a major title, fired nine birdies on the way to a 16-under par total of 200.
Defending champion Patty Tavatanakit, playing in the same group, had six birdies in a two-under par 70 for 206, her birdie at the 18th putting her one stroke in front of American Jessica Korda, who carded a five-under par 67 for 207.
"It was a really fun round out there," said Kupcho, a 24-year-old who won the inaugural Augusta National Women's Amateur three years ago.
She got off to a hot start in the California desert, rolling in birdies at the second, third, fourth and fifth holes to reach 12-under four the tournament.
She added four more birdies at the eighth, 10th, 11th and 12th and looked poised to challenge for the course-record of 62 set by Lorena Ochoa in 2006 and matched by Lydia Ko last year.
She gave a stroke back with her lone bogey of the day at 13, but rolled in one last birdie at the 17th before tapping in for par at the last.
"It was nice to see the putts start falling after yesterday," Kupcho said.
Although she said she was excited about her career-best round, as it was happening it was "all a blur."
"You're just out there grinding shot by shot," Kupcho said.
Patty couldn't keep pace, but the Thai star whose dominant victory last year helped propel her to Rookie of the Year honours, made sure she'll play in the last group on Sunday with her closing birdie.
She's trying to become just the third player, after Sandra Post in 1978-79 and Annika Sorenstam in 2001-02, to win back-to-back titles at Mission Hills.
But the outcome Sunday is now in Kupcho's hands.
"I'm not going to think about it," she said of her chance at making her first LPGA title a major. Since turning pro she has two runner-up finishes and 11 top-10s.
She said she didn't look at scoreboards much on Saturday, and she wouldn't on Sunday either.
"I can't control what anyone else does," she said. "Someone could go out and do what I did today, props to them if they do. I'm just going to go out and play my own game."
Although Korda enjoyed an impressive day, watching Kupcho complete her round she was under no illusions of what it might take to overhaul her compatriot on Sunday.
"A lot lower than what I shot today," Korda said.
Overnight leader Hinako Shibuno of Japan's bid for a second major title to go with her 2019 Women's British Open crown dissolved in a disastrous third-round 77 that featured two double bogeys and left her 12 shots adrift.
THIRD ROUND SCORES:
200- Jennifer Kupcho 66-70-64
206- Patty Tavatanakit (THA) 67-69-70
207- Jessica Korda 71-69-67
209- Annie Park 69-67-73
210- Brooke Henderson (CAN) 72-71-67, Ryann O'Toole 73-69-68, Hannah Green (AUS) 70-72-68, Lexi Thompson 69-70-71, Gabriela Ruffels (AUS) 68-71-71, Nanna Koerstz Madsen (DEN) 71-67-72, Kim Hyo-joo (KOR) 70-67-73
211- Yuka Saso (JPN) 75-70-66, Atthaya Thitikul (THA) 74-68-69, Charley Hull (ENG) 71-71-69, Madelene Sagstrom (SWE) 71-70-70, Lydia Ko (NZL) 68-73-70, Pia Babnik (SLO) 70-70-71, Sarah Schmelzel 69-71-71, Celine Boutier (FRA) 70-69-72, Kim Sei-young (KOR) 70-67-74
212- Stephanie Meadow (NIR) 73-72-67, Pajaree Anannarukarn (THA) 68-71-73, Caroline Masson (GER) 68-71-73, Minjee Lee (AUS) 66-73-73, Hinako Shibuno (JPN) 69-66-77