Lydia Ko fired an opening round five-under-par 67 on Thursday to take a one-stroke lead at the Swinging Skirts LPGA Classic, where she recorded her first win as a professional last year.
World number one Ko, on the eve of her 18th birthday, made six birdies with just one bogey at the Lake Merced Golf Course in San Francisco.
Hot on Ko's heels are three players at four-under, including South Korean rookie Jang Ha-na and 54-year-old Juli Inkster, a winner of 31 LPGA tournaments and seven majors but who has not won in nine years.
Americans Stacy Lewis and Morgan Pressel shot 69s to finish at three-under.
World number two Park In-bee had a tough start to the tournament with four bogeys en route to a one-over 73 while fellow South Korean Kim Sei-young, who holed out in Hawaii to win last week's LOTTE championship, shot a two-over 74.
Michelle Wie finished at three-over.