UPDATE 1-Golf-Choi leads Ochoa, Kerr by one at U.S. Women's Open

* Choi leads by one shot over Ochoa and Kerr

* Big names struggle on tough pin placements

By Larry Fine

BETHLEHEM, Pennsylvania, July 9 (Reuters) - Choi Na-yeon of South Korea fired a three-under-par 68 to take a one-shot lead over world number one Lorena Ochoa and 2007 winner Cristie Kerr after the first round of the U.S. Women's Open on Thursday.

Choi, who began her round on the 10th at Saucon Valley, birdied her first three holes, sticking two of the approach shots within two feet.

The 21-year-old Korean, who posted nine top-10 finishes in her rookie season last year, holed out from a bunker on her 11th hole and drained a 60-foot birdie putt at the next to reach five-under before dropping back with a pair of bogeys.

"This is the biggest tournament of our schedule on the LPGA," Choi said through an interpreter. "I started well today, but I'm not going to let it slide.

"I'm going to make sure that I play my best to the end."

Mexico's Ochoa, Kerr and unheralded fellow-American Jean Reynolds, 24, were tied for second at two-under 69 on a challenging 6,740-yard layout that featured fast, sloping greens and devilish pin placements.