Mirim Lee struck a blemish-free opening round 65 to lead the Kia Classic, with world No 1 Lydia Ko two strokes back.
The two-time LPGA winner fired seven birdies to hold the clubhouse lead ahead of former winner Yani Tseng, who carded an unlikely gain on her final hole when her impressive approach shot from a difficult stance in the rough left a simple six-foot putt.
Ko, chasing a third win of 2015, reached the turn in 33 and picked up two further strokes on her back nine to move five-under alongside Australia's Karrie Webb and China's Xiyu Lin, with world No 2 Inbee Park and America's Lexi Thompson part of a ten-player group on four-under.
"I'm coming in with a lot of confidence and know that my game is there," Ko told reporters. "It's always good to know that you can be aggressive and just to trust myself.
"I've been playing pretty solid the last couple of weeks. It's a pretty cool streak."
US Open champion Michelle Wie carded five birdies on her front nine to challenge the leaders, but ended the opening round 69 after posting a third bogey at the 18th.
Over 40 of the world's top 50 are in action this week in a high-profile field in California, with defending champion Anna Nordqvist opening level par.