Belarusian seventh seed Aryna Sabalenka improved her chances of reaching the WTA Finals last four with a 6-3 7-5 win over American Jessica Pegula.
Sabalenka, 24, can only be stopped from qualifying by Tunisia's Ons Jabeur winning in straight sets against Greece's Maria Sakkari later on Friday.
Any other result - a Sakkari victory or Jabeur winning in three sets - sees Sabalenka through for the first time.
Fifth seed Sakkari, 27, is already assured of progressing as group winner.
The season-ending event sees the world's eight best players this season split into two groups of four.