Tseng won with routine par when Pressel failed to get up and down out of a greenside bunker. In 2001, Meredith Duncan became the most recent player to win both prestigious events in the same year.
The 17-year-old Pressel fell 3-down early in the afternoon round before rallying to square the match by winning three consecutive holes. Tseng regained the lead with a birdie on the par-5 10th.
At No. 16, Pressel eagled the par 5 to pull even again, then missed a 20-footer for birdie on the final hole that would have won the title.
Pressel won the U.S. Women's Amateur a week ago in a rout, beating college player Maru Martinez 9-and-8 at Settindown Golf Course north of Atlanta. She likely won't defend that title, since she has applied to join the LPGA Tour and will be allowed to attend qualifying school this fall.
If Pressel makes it through Q-school, she'll get her tour card next spring after turning 18.