The 28-year-old Spaniard beat Anett Kontaveit 6-3, 7-5 to win WTA Finals title for the first time in her career and improve her record playing in Mexico to 14 wins and two losses.
In a contest between two former No.1-ranked players, the third-seeded Pliskova struggled early adjusting to the nearly 5,000-feet altitude of Guadalajara, Mexico's second largest city on Wednesday.
Kontaveit turned the second set around from 4-0 down and mounted another comeback to win the decider with a break of serve when Alexandrova served for the match at 5-4 up.
Anett Kontaveit entered the tournament on a four-match losing streak, then won five matches in six days in the inaugural WTA 250 event.
Jelena Ostapenko will meet Estonia's Anett Kontaveit in the women's final at Eastbourne after an impressive win against Elena Rybakina.