Czech Pliskova came through her semi-final against Timea Bacsinszky, who had ended the WTA Finals hopes of Aryna Sabalenka in the last eight, 6-2 6-1.
Garcia moved into Sunday's final with a 6-3 6-4 win over fifth-seeded Hsieh Su-wei.
Teenager Dayana Yastremska easily saw off the challenge of Wang Qiang in the Hong Kong Open final to win her first WTA Tour trophy.
The world No. 16 took out the top seed 7-6 (7), 6-3.
Former top-10 player Petkovic looked set to make her first final since 2015 but 23-year-old Alexandrova, ranked 119, mounted a superb comeback to triumph 0-6 6-4 6-0.
Now in eighth place in the race to Singapore, the Czech will have to wait out the results from next week's tournaments in Luxembourg and Moscow to see if she qualifies for one of the two remaining spots.
The Czech broke back to 5-2 at the end of a tense battle in that game with Pliskova squandering the first two break points, Garcia giving a third opportunity with a double fault and missing the first chance to serve out the match.