San Martin - South Korea's Lee Mi-Rim, also known as Mirim Lee, stormed into the outright lead by three strokes at the US Women's Open, firing a record-tying eight-under 64 in Thursday's opening round.
The world No 32 tamed the CordeValle Golf Club course to become the first player since 1999 in the US Women's Open history to shoot a round of eight under.
"The course is perfect now," Lee said. "The greens are really fast. I just try make par that's it."
Lee came within one shot of tying Helen Alfredsson's record for the lowest 18-hole score in the tournament of 63, set in the first round in 1994 in Michigan.
Lee owns a three-shot lead over Cristie Kerr, Min-Jee Lee and Amy Yang and a four-shot lead over Kelly Tan, Brittany Lang and Anna Nordqvist.
Kerr said she expects course conditions to get tougher as the week goes on.
"We played in the morning and took advantage of it," Kerr said of the cool weather and softer greens.
"Obviously, Mi-Rim had almost a career round today. I definitely expect it to change.
"I don't think the USGA likes somebody shooting eight under on their golf course. So you have to expect it to change, and if it doesn't, then you'll have opportunities to score.
"It's a very straightforward course."
Kerr, who won the US Women's Open in 2007, had seven birdies and two bogeys. Australia's Min-Jee Lee had an eagle on the par-four sixth, four birdies and one bogey. Yang had six birdies and one bogey.
Leading scores after Thursday's opening round of the US Women's Open (par-72, USA unless stated):
64 - Lee Mi-Rim (KOR)
67 - Amy Yang (KOR), Cristie Kerr, Min-Jee Lee (AUS)
68 - Anna Nordqvist (SWE), Brittany Lang, Kelly Tan (MAL)
69 - Ji Eun-Hee (KOR), Moriya Jutanugarn (THA), Sydnee Michaels
70 - Jessica Korda, Gerina Piller, Park Sung-Hyun (KOR), Alena Sharp (CAN), Jodi Ewart Shadoff (ENG), Lizette Salas, Haru Nomura (JPN), Pornanong Phatlum (THA), Ariya Jutanugarn (THA), Christina Kim, Pernilla Lindberg (SWE), Park Hee-Young