(Reuters) - South Korea's Lee Mi-rim birdied five of her last seven holes to cap an "easy day" and surge into an early three-stroke lead in Thursday's opening round of the U.S. Women's Open in San Martin, California.

Taking advantage of ideal scoring conditions at sun-splashed CordeValle Golf Club, the 25-year-old fired an eight-under-par 64 to match the second-best U.S. Women's Open score ever in the third of the year's five women's major championships.

Only Swede Helen Alfredsson, with an opening 63 at the 1994 U.S. Women's Open, has gone lower while Americans Kelli Kuehne and Becky Iverson, Canadian Lorie Kane and South Korean Chella Choi have also carded 64s at the tournament.

"Everything was good, like driver is good and then (my) irons were good," a beaming Lee told reporters after covering her back nine in a sizzling five-under 31. "Putting, a couple of missed putts, but not bad. It's good ... just good play.

"The course is perfect now."

Asked if she had hit her approach shots close on most of the holes, Lee replied: "Yeah, so today's round was an easy day ... an easy round."

American Cristie Kerr, the 2007 U.S. Women's Open champion at Pine Needles, and Australia's Minjee Lee carded 67s, while Swede Anna Nordqvist, Malaysia's Kelly Tan and American Brittany Lang opened with 68s.

LPGA veteran Kerr relished playing the opening round in the company of the pacesetting Lee.

"Yeah, it definitely helps," said the 38-year-old Kerr, who has won 18 LPGA titles, including two majors. "And it was fun. If you looked at Mi-rim's card and my card, we might have shot a 59 best ball, or close to it.

"You would expect the USGA (United States Golf Association) is not too happy with that score, so it will be interesting what they come up with (for course set-up) for the rest of the week."

South Korea's world number six Chun In-gee, who won last year's U.S. Women's Open at Lancaster Country Club by one shot, launched her title defence with a one-over 73.

New Zealand's world number one Lydia Ko and second-ranked Canadian Brooke Henderson were among the late starters on Thursday in a high-profile grouping completed by fourth-ranked American Lexi Thompson.

(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Larry Fine)