Andujar, ranked No. 71, beat fourth-seeded Fernando Verdasco 6-3, 6-7 (2), 6-1 in an all-Spain semifinal at the clay-court Roy Emerson Arena.
Argentina's Monaco rallied from losing the first three games to win 6-3, 7-6 (4) against seventh-seeded Robin Haase of the Netherlands.
Monaco, now 105th-ranked two years after peaking at No. 10, will seek his ninth career title Sunday, and Andujar his third.
Andujar wasted a winning chance in the second set when, two points from victory at 5-3, he sent down back-to-back double faults.
However, he dominated the decider with three service breaks.