SAN FRANCISCO - Free-swinging Park Sung-Hyun fired seven birdies in a six-under par 66 on Friday to grab a two-stroke lead after two rounds of the US Women's Open at CordeValle in San Martin, California.

Park, playing in her first US Open, took advantage of benign morning conditions and vaulted to the top of the leaderboard on eight-under 136, two strokes in front of compatriots Amy Yang and Lee Mi-Rim with world number one Lydia Ko lurking just a further stroke back. "I didn't even think about winning because this is the first time for me," said the 22-year-old Park, a five-time winner on the Korean LPGA Tour who has played three LPGA Tour events this year. "I would like more experience with the USGA, LPGA. But I'm trying to enjoy this tournament. That's why I am just more comfortable, don't even think about the winning, I just enjoy the play. That's why it happened today."

Park's seven birdies included four in six holes on the back nine. Her lone bogey came at the 17th, where she was unable to get up and down from a greenside bunker. She was also in trouble at 18, finding a hazard off the tee but rolling in a 15-footer to save par and walk off the green tied with overnight leader Lee -- whose second-round 74 was 10 strokes higher than her record-equalling opening round of eight-under.

Lee's difficult day included a double-bogey six at the par-four 10th, but she stayed in touch alongside Yang, who carded a 71. New Zealand's Ko rebounded from a one-over par first round, shaking off a bogey at the first to grab seven birdies the rest of the way -- including four in a row starting at the third.

She was tied on 139 with Japan's Haru Nomura, who had four birdies in her 69. "I got some putts rolling," Ko said. "My birdie on three kind of turned the round around and making the string of birdies definitely helped. "Just to know that this is the first time I was under par for the tournament kind of put myself in a positive position. I just tried to enjoy it out there."

Still just 19, Ko is seeking a third major title. Last year she became the youngest woman to win a major with her triumph at the Evian Championship, and she added her second at the ANA Inspiration this year. She came close again in the Women's PGA Championship last month, falling in a playoff to Canadian phenom Brooke Henderson. "I think the more I play, I think the more I get used to it, especially at these majors where the courses are tough," Ko said. "You need to stay patient. I think that's the big key I've been learning, just keep my head high and just enjoy it out here."

Henderson, 18, was grouped with Ko in a battle of teen titans in the first two rounds. She followed up a first-round 76 with a one-under 71 that left her three-over, part of a big group that also included world number four Lexi Thompson. Thompson, who played with Ko and Henderson, signed for a 73.