Polish second seed Iga Swiatek moved within touching distance of the WTA Finals last four after a confident win over US Open champion Coco Gauff.
Four-time major champion Swiatek, 22, won 6-0 7-5 in a a windy Cancun and has now won both of her group matches.
Tunisia’s Ons Jabeur beat seventh seed Marketa Vondrousova 6-4 6-3.
Jabeur, who lost to the Czech in this year’s Wimbledon final, joins Gauff with one win and one loss and plays Swiatek in her final group match.
The pair last met in the US Open final, when Swiatek triumphed.
“Iga is a smart person and she knows how to adapt to these conditions,” said Jabeur.
Both players had seven break points in a tight opening set but it was Jabeur who held out after breaking for a second time for a 3-2 lead.
And, after an exchange of breaks early in set two, Jabeur broke to love in the seventh game before clinching victory on Vondrousova’s serve two game later.
Vondrousova, despite losing both of her matches, could still yet qualify with victory over Gauff in her final match.
Gauff, 19, served for the second set at 5-4 but hit four double faults in a row to gift the break back to Swiatek.
Third seed Gauff, who can still qualify after thrashing Jabeur, won one more point as Swiatek sealed victory.
“Coco is the kind of player who doesn’t give up so I knew it wouldn’t be easy,” said Swiatek, who is aiming to regain her world number one ranking from Aryna Sabalenka at the season-ending event.
On Tuesday, American Jessica Pegula beat top seed Sabalenka in the other group to become the first player to reach the semi-finals.
Belarus’ Sabalenka plays Kazakhstan’s Elena Rybakina on Thursday to determine the second player to qualify from the group.
The Finals have been marred by the players criticising the court conditions at the venue, where the construction of the surface was only completed in the days before the tournament started on Sunday.