Sung Hyun Park shot a six-under round of 66 to take a two-shot lead into the weekend at the US Women's Open in San Martin, California.
Korea's Park sits on eight under for the tournament, clear of compatriots Mirim Lee and Amy Yang.
"This is the first time in a USGA tournament. And coming to the tournament, I didn't even think about winning because this is the first time for me," Park said.
"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."
Lee went into the second round with a three-shot lead but quickly dropped a couple of shots with bogeys at the third and fourth.
She recovered with three birdies in four holes but a double bogey at the 10th and further dropped shots on the 12th and 14th set her back before a birdie on the 15th saw her sign for a two-over 74.
Yang held a share of the lead but two bogeys in her final three holes dropped her back.
World number one Lydia Ko bounced back from an opening 73 with a six-under 66 to move into a tie for fourth place on five under alongside Haru Nomura.
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.
Catriona Matthew posted a 72 to make the cut on 145 but fellow Scot, Pamela Pretswell, making her US Open debut, missed the mark by two shots on a 150 after a 75.
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