The 19-year-old New Zealander rolled in a putt to birdie the 18th hole, which leaves her with a one shot lead on seven-under-par.

Ko, who is hoping to win a third major title, placed herself in an ominous position yesterday with a round of 66 that improved on her opening day 73.

Her 70 today was enough to edge her in front of two South Koreans, yesterday's leader Sung Hyun Park and Ji Eun-hee, by a single shot.

Park fired a 74 which was eight shots worse than the 66 which saw her claim top spot yesterday.

The best round of the day was produced by American Brittany Lang, whose 68 lifted her up to five-under where she is part of a four-way tie.

"The biggest difference today was the wind," said South-Korean born Ko, who won last year's Evian Championship and the ANA Inspiration this year.

"I knew it would play a lot tougher than yesterday because the majority of my round yesterday was played in calm weather.

"It was really tough out there because the course is drying up, firming up, and it was tough to get near the pins on some holes."

The US Women's Open will conclude with the final day tomorrow.