Ernst set the stage as the first player on the course. Playing by herself, the tour rookie birdied eight of her first 10 holes to flirt with a 59, then bogeyed the par-4 11th. She is 12 under overall.
Tseng, who opened with rounds of 67 and 68, had 11 birdies and two bogeys in warm, dry and calm conditions to reach 18 under.
Tseng birdied six of the first eight holes and dropped a stroke on the par-4 ninth to make the turn in 5-under 31. She birdied No. 10, then hooked her second shot up against a restroom en route to a bogey on the par-5 12th.
Tseng rebounded with a birdie at 13, and added birdies at 15, 16 and 17 all with putts of less than 10 feet to take the lead. She squandered a chance for another birdie when she missed a 5-foot putt on the par-4 18th.
