Seventh-seeded Lesia Tsurenko booked a WTA title showdown with France's Kristina Mladenovic in Acapulco after illness forced top-seeded Mirjana Lucic-Baroni to retire while trailing against the Ukrainian.
Croatian veteran Lucic-Baroni, a semi-finalist at the Australian Open, had been in strong form all week, reaching the semi-finals without dropping a set.
But she looked out of sorts from the start against Tsurenko, who raced to a 5-0 lead before the top seed retired complaining of gastrointestinal illness.
"I was really shocked on court when she decided to retire," Tsurenko said. "I wish that she feels better. I'm pretty happy to be in the final, but it's a shame that she's not feeling well."
Lucic-Baroni was dismayed with the end to her week.
"I was really unlucky," she said. "I picked up a virus, I vomited just before I went on the court.
"I tried. Because I'm not the type to retire, ever. I'll just put on more tapes and play, and I can play through anything really.
"But I just felt extremely dizzy and like I was going to vomit all the time, and I just couldn't continue. I'm really upset, because this is such a great tournament and I'm playing so well."
Mladenovic made it into her second final of the year, holding off fifth-seeded American Christina McHale 7-5, 4-6, 6-2.
The two traded breaks to start the final set, but Mladenovic took charge with a strong service performance and powerful forehand. She won three straight games to close out the match to give herself a chance to add another title to the first career trophy she lifted in St. Petersburg last month.