South Korea's Sung Hyun Park fired a six-under-par 66 to take a two-shot lead at the halfway stage of the US Women's Open.
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 to storm to the top of the leaderboard at CordeValle Golf Club in San Martin, California.
She bogeyed her penultimate hole - the par-four 17th - but her eight-under total of 136 put her two clear of fellow Koreans Amy Yang (71) and first-round leader Mirim Lee, who could only manage a two-over-par 74.
Park said: "Coming to the tournament, I didn't even think about winning because this is the first time for me. I'm trying to enjoy this tournament. That's why I am comfortable, don't even think about the winning."
World No 1 Lydia Ko of New Zealand, who is seeking her third major title at the age of 19, followed her opening 73 with a seven-birdie 66 to finish just three shots off the pace.
Ko's two playing partners in a marquee group, Canadian world No 2 Brooke Henderson (71) and fourth-ranked American Lexi Thompson (73), each ended the day at three over to narrowly make the cut which came at four over.
England's Jodi Ewart Shadoff is the leading European after a one-under 71 saw her into a tie for 10th place on three under, while Scotland's Catriona Matthew is joint-29th on one over following a 72.
However, Charley Hull will have the weekend off after a second-round 75 left the Englishwoman on five over.