Marisa Baena, the No. 60 seed, clinched her fifth upset of the tournament on Sunday when she bested No. 8 seed Candie Kung, 2-up, in one semifinal.
Baena will face Meena Lee, the 47th-ranked player, in the final. Lee advanced thanks to a 1-up victory over American Wendy Ward, No. 14 in this championship.
Both Baena and Lee will be in search of their first victory on the LPGA Tour. Also at stake is $500,000, the richest first-place prize in LPGA Tour history.
Kung, who erased a 2-down deficit with four to play against No. 1 Annika Sorenstam in Saturday's quarterfinals, went 2-up thanks to a par at No. 12 and a birdie at the 13th.
Baena knotted things up with a birdie at the par-5 14th and a par at No. 16. Kung bogeyed 18 to drop the match to the Colombian, who ousted Natalie Gulbis (No. 5), Grace Park (No. 37), Jennifer Rosales (No. 21) and Karrie Webb (No. 29) en route to the semifinals.
Ward won the eighth hole to go 1-up, then the pair halved the next seven. Lee won the 16th with par to square the contest, then parred 18 to move into the final.
Lee defeated Hee-Won Han (No. 18), Kim Saiki (No. 50), Liselotte Neumann (No. 31) and Pat Hurst (No. 39) to make the final match.