Jessica Pegula progressed to the last four at the WTA Finals with a perfect record but Aryna Sabalenka and Elena Rybakina must resume their match on Friday after play was suspended.
American Pegula had already qualified but the fifth seed made it three successive victories in Cancun by beating Greek Maria Sakkari 6-3 6-2.
World number one Sabalenka led 6-2 3-5 against Rybakina, who will serve to level the match when play resumes.
The winner will reach the semi-finals.
Also on Friday, Iga Swiatek faces Ons Jabeur, while Coco Gauff plays Marketa Vondrousova to determine who will progress from the Chetumal Group.