Pornanong Phatlum carded a 6-under 66 on Friday and remained one stroke clear of the field after two rounds of the Safeway Classic.
Phatlum is going for her first official LPGA Tour victory. She did win the unofficial event in Brazil last year.
Suzann Pettersen fired a 9-under 63 in round two to jump into second place at minus-13. Sandra Gal posted her second straight 66 and is alone in third at 12-under-par 132 at Columbia Edgewater Country Club.
Caroline Masson is alone in fourth at minus-11 after firing a 64 in round two.
Cristie Kerr (68), Sandra Changkija (66) and Lizette Salas (68) share fifth at 10-under-par 134.
MORE TO FOLLOW.