(Reuters) - South Korea's Park Sung-hyun charged into an early two-shot clubhouse lead in the second round of the U.S. Women's Open on Friday with the ominous figure of world number one Lydio Ko in hot pursuit.

Park, who has three wins on the Korean Tour and is playing her first season on the LPGA circuit, birdied seven of her first 16 holes on the way to a sparkling six-under-par 66 at CordeValle Golf Club in San Martin, California.

Though she bogeyed her penultimate hole, the par-four 17th, her eight-under total of 136 put her two strokes clear of her closest competitor, fellow Korean Amy Yang who carded a 71.

New Zealander Ko, at the age of 19 seeking her third major title, followed her opening 73 with a seven-birdie 66 to surge up the leaderboard, finishing the day just three shots off the pace with half the field back in the clubhouse.

"I didn't start off very well, missing the fairway on one," Ko told reporters after beginning her round with a bogey and ending it with a four-foot putt for birdie at the last.

"But my birdie on three kind of turned the round around and making the string of (four consecutive) birdies definitely helped.

"I'm playing on the Tour that I've always dreamt of playing, and this is the biggest championship in the U.S." said Ko, who won her first major title at last year's Evian Championship before adding a second at the ANA Inspiration in April.

"There's a lot of golf to be played, so I don't really want to get ahead of myself."

Ko's two playing competitors in a marquee group, Canadian world number two Brooke Henderson (71) and fourth-ranked American Lexi Thompson (73), were both flirting with the projected cut after ending the round at three-over.

Korean Park, who made her LPGA Tour debut at the Founders Cup in March, said she had benefited from a strategy of simply enjoying herself on the course.

"Coming to the tournament, I didn't even think about winning because this is the first time for me," she said of her maiden U.S. Women's Open.

"I'm trying to enjoy this tournament. That's why I am comfortable, don't even think about the winning."

South Korea's Lee Mi-rim, who led by three shots overnight after opening with an eight-under 64, was among the late starters on Friday.

(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Andrew Both)