Tseng played the front nine of the Locust Hill Country Club course in 5-under 30 and overcame two bogeys on the back side.
Paula Creamer was alone in second at 67 while those tied for third at 68 included Diana D'Alessio, Stacy Prammanasudh, Angela Stanford and Meena Lee.
Stacy Lewis, who won the year's first major event at the Kraft Nabisco Championship, was among those at 3-under 69.
Defending champion Cristie Kerr, second behind Tseng in the world rankings and the runner-up in each of the last three tour events, shot an even-par 72 and was tied for 33rd place. Kerr won last year's LPGA Championship by 12 shots.
This week begins a stretch of three major tournaments in four events for the best players in women's golf. The U.S. Women's Open is scheduled to begin July 7 in Colorado Springs. The Women's British Open will be played three weeks later in Carnoustie, Scotland.