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 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.