Angelique Kerber and Flavia Pennetta qualified for the WTA Finals when Lucie Safarova and Karolina Pliskova were knocked out of the Kremlin Cup on Wednesday.
Kerber withdrew from the tournament in Moscow with a back injury which troubled the world number seven in her Hong Kong Open final defeat to Jelena Jankovic on Sunday.
The German's absence did not cost her a place in the season-ending tournament in Singapore, though, as defending champion Anastasia Pavlyuchenkova sent Safarova packing with a 6-1 6-7 (2-7) 6-3 victory.
Second seed Safarova has now lost both games since recovering from a bacterial infection that had kept her out since the US Open, struggling on serve as she racked up seven double faults and was broken six times.
Pavlyuchenkova won her first WTA title of the year in Linz at the weekend and maintained her momentum to stay in the hunt for back-to-back Kremlin Cup titles with a battling display in which she saved seven break points.
US Open champion Pennetta later became the seventh player to be guaranteed a place in the WTA Finals when sixth seed Pliskova lost 6-3 7-6 (10-8) to Anastasija Sevastova.
Pliskova had to win the Kremlin Cup to be in with a chance of playing in Singapore, but bowed out in the second round after Pennetta had beaten Daria Gavrilova 6-2 6-4.
Carla Suarez Navarro and Safarova are the only two players in contention to seal the eighth and final place in the WTA Finals, with the Spaniard needing to reach the final to qualify.
Qualifier Daria Kasatkina is through to the third round after getting the better of seventh seed Irina-Camelia Begu, while Margarita Gasparyan and Lesia Tsurenko were also second-round winners.