Tseng, ranked No. 1 in the world, had a two-day total of 135 at the La Costa Resort and Spa as she seeks her third victory in five starts this year.
LPGA Hall of Fame member Se Ri Pak shot a 66 to equal the low round of the tournament and moved into second place alone at 137.
Tseng has shot in the 60s in 10 of her last 13 rounds on the tour and she swept to a victory last week at the LPGA Founders Cup after opening with a 65. She had seven wins in 2011, including triumphs at the LPGA Championship and Women's British Open.
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