World number one Yani Tseng carded a four-under 68 to head the Kraft Nabisco Championship after the second round.
The Taiwanese reached eight-under to lead Haeji Kang by one shot at the first LPGA Tour major of the season.
Despite a one-over second round, England's Jodi Ewart's was in joint 16th place on two-under.
Charley Hull was tied in 69th in California after a 77 left the world number seven ranked amateur on four-over at her first professional event.
"You never get tired when you want to win a tournament," said Tseng, who is chasing her third consecutive LPGA tournament victory.
"I just figured out I'm tired after the last two wins when you come into this week, but after [Thursday], I had a good sleep, and I relaxed a few days, and I think I'm coming back with a fresh focus."