Park Hee-young, who is coming off a missed cut at the US Women's Open, shot a 10-under 61 to take the lead in the third round of the Manulife Financial Classic.
South Korea's Park is at 20-under par 193 and has a slim one-shot lead heading into Sunday's final round of the LPGA Tour event at the Grey Silo Golf Course.
Park is just the 11th player in LPGA history to shoot a 61.
"I didn't realize it," Park said. "My long putting was pretty awesome. I just tried to trust it."
American Angela Stanford, who is aiming for her sixth LPGA Tour win, remained in second place with a third-round 64.
Stanford is 19-under-par 194, one shot ahead of Scotland's Catriona Matthew, who shot 68 and fell into third place at 18-under.
Meena Lee (65) is alone in fourth at 17-under, while Swede Anna Nordqvist (67) rounds out the top five at 15-under.
Gerina Piller, of the US, fired a 62 and vaulted into a tie for sixth at 14-under. She is level with South Korea's Amy Yang (66) and American Austin Ernst (67).
World number one Park In-bee, the winner of the last three LPGA events, shot 68.
Park, who also won the first three majors of 2013, is tied for ninth place with Norway's Suzann Pettersen (68) at minus-13.
Park has her work cut out for her if she is to win her fourth straight event.
"I had a lot of opportunities today but missed a lot of putts," she said.