Angela Stanford carded a five-under par 67 to secure a two-stroke lead over South Korea's Inbee Park after the second round of the Lorena Ochoa Invitational in Mexico.
Stanford, who won the inaugural tournament in 2009, has an 11-under 133 total at Guadalajara Country Club.
Park went round in 68, while Cristie Kerr was third at eight-under with a 69.
Second-ranked Stacy Lewis, competing on the back of her tour-leading fourth victory of the season in Japan, finished Friday seven-under along with So Yeon Ryu and Candie Kung.
But America's Michelle Wie, who was tied for the lead after the opening round, suffered a disastrous round as she was forced to settle for a 75 to drop into a tie for 12th place at three-under.
Top-ranked Yani Tseng was also three-under after a 71, while Ochoa was back on one-under after a 72.