Morgan Pressel shot a two-under-par 70 on Saturday to seize a two-stroke lead going into a 36-hole Sunday finish at the weather-delayed LPGA Championship at Locust Hill outside Rochester, New York.
The short-hitting Pressel, looking for her first LPGA Tour win in five years and her second major championship title, relied on her accuracy on a course playing extremely long after heavy rains, stood at six-under-par 138.
"I just hung in there," said Pressel. "I feel calm out there."
Tied for second behind the American were world number one Inbee Park and her South Korean compatriot Chella Choi.
Park, winner of three tournaments this year including the season's first major, the Kraft Nabisco, made five birdies and a bogey in firing a 68, while Choi registered 73 for 140.
Another shot off the pace were South Koreans Jiyai Shin (73), the Women's British Open champion, and Amy Yang (70) and Australian Sarah Jane Smith, who posted a 69 to move up the leaderboard.
Defending champion Shanshan Feng of China bounced back from her first-round 74 with a 70 that put her at level par 144.
Lightning and heavy rains scrubbed Thursday's scheduled opening round, setting up the marathon finish on Sunday.
Pressel, starting at the 10th hole, surged to a three-shot lead with birdies on three of her first six holes before giving two strokes back with bogeys at the 16th and 18th.
The 25-year-old, who became the youngest ever winner of a women's major when claimed the Kraft Nabisco major in 2007 as an 18 year old, improved her lead by a second stroke with a birdie on her penultimate hole.
"I feel like I'm hitting the shot that I'm seeing and can put the ball where I want to," said Pressel, who has been bothered by injuries in recent seasons.
"We got a marathon day tomorrow and hopefully I see a lot of really good shots in my head and execute them all."