SAN FRANCISCO (AFP) - World No. 1 Lydia Ko, seeking to add a third Major title to her resume, birdied the 18th to take the outright lead after three rounds of the US Women's Open on Saturday.

The 19-year-old New Zealander had three birdies and one bogey on a blustery day on the CordeValle course in San Martin, California, her two-under 70 giving her a seven-under total of 209 and a one-stroke lead over South Koreans Park Sung Hyun and Ji Eun Hee.

Park, a five-time winner on the Korean LPGA Tour who is playing in her first US Open, held a two-stroke overnight lead.

She carded a two-over par 74 to join Ji, who signed for a 70, on 210.

South Korean Amy Yang carded a one-over 73 to share fourth place with American Brittany Lang (68) on 211.

Ko, who won the Evian Championship last year and the ANA Inspiration this year to take her Major tally to two at the age of 19, had surged into contention with a 66 on Friday.

She said conditions were less conducive a day later.

"It was really tough out there," Ko said. "The course is drying up, firming up. It was tough to get near the pins on some holes."