Tseng tied Stacy Lewis for the top spot in the 54-hole tournament at 5-under 66. Neither player made a bogey.
Lewis, who has climbed to No. 11 in the world rankings, is seeking her first victory since capturing the Kraft Nabisco Championship in April.
Jin Young Pak, Karen Stupples and Taylor Leon shared third place at 67. Leon has never finished higher than sixth in a tour event.
Suzann Pettersen, ranked second in the world behind Tseng, was among those at 68. She was joined at that number by Morgan Pressel and Paula Creamer.
Third-ranked Cristie Kerr, Angela Stanford and Ai Miyazato were part of the group at 2-under 69.
Tseng had seven 3's on her scorecard, including her eagle at the par-5 closing hole at the Pinnacle Country Club.