Rosales rolled in a 10-foot birdie putt at the final hole for a 4-under 67 in calm conditions at The Orchards course.
That put her at 5-under 137 halfway through the tournament, a shot in front of Candie Kung (68) and Kim Saiki (68).
Michelle Wie, given a special exemption to play in the tournament at age 14, was four shots back after a 70.
"I played great today," Wie said after making six birdies, three bogeys and a double bogey.
Rosales, 25, is a former amateur star who won the NCAA individual championship while a freshman at the University of Southern California. She recorded her first LPGA Tour victory earlier this year.