The last day of Group El Tajín round-robin play ended with the two highest-seeded teams going through to the semifinals after No.1 Barbora Krejcikova and Katerina Siniakova came back from the brink to defeat No.6 Alexa Guarachi and Desirae Krawczyk 5-7, 7-6(3), [10-7]; and No.3 Hsieh Su-Wei and Elise Mertens came through 6-4, 7-6(3) over No.8 Sharon Fichman and Giuliana Olmos.
Following Krejcikova and Siniakova's win in the first match, Fichman and Olmos went into their third night session needing to win in straight sets, while Hsieh and Mertens required only one set. The Wimbledon champions made a solid start and led all the way in the first set, though sealing it wasn't straightforward.
Twice they went up a break, with Mertens coming up with the goods on big points: the Belgian struck a backhand winner to break Fichman in the third game, and hammered a return to capture the Olmos serve in the fifth game. However, Hsieh twice dropped serve to open the door for a Fichman/Olmos comeback that would have been cheered all the way by the Mexican crowd.
Mertens would again step up to the plate when needed, closing out the set to love and sealing it with a service winner. With the semifinals fixed, the second set was meaningless in terms of advancement - though with Fichman and Olmos battling hard to avoid going 0-3 in their WTA Finals debuts, it was packed full of quality all-court points.
In the event, Hsieh and Mertens were too solid in the tiebreak. Another Mertens service winner sealed the win, and a last-four date with No.2 seeds Shuko Aoyama and Ena Shibahara.
Earlier, Krejcikova and Siniakova had already guaranteed their progress into the semifinals, but needed to win to assure their top spot in the group regardless of the second result. Guarachi and Krawczyk, meanwhile, required victories for both themselves and Fichman/Olmos.
The Chilean-American underdogs left it on the line, delivering a sterling effort to edge a tight opening set and come within two points of victory at 5-4 in the second. But Krejcikova and Siniakova showed all their experience to extricate themselves from danger, dominate the second-set tiebreak and keep their noses in front throughout the match tiebreak.
The Czech duo will face No.4 seeds Nicole Melichar-Martinez and Demi Schuurs in the last four, where they will be bidding to reach their second final at the WTA Finals after being runners-up in 2018.