The top-seeded Pliskova beat Polona Hercog 6-1, 6-2, while the fourth-seeded Sabalenka defeated Magda Linette 6-1, 6-3.
Pliskova, who saved the one break point she faced in the final game of the match, is 4-1 against Hercog and now in eighth place in the race to qualify for Singapore.
Sabalenka is trying to qualify for the year-end WTA Finals for the first time.
Also, fifth-seeded Hsieh Su-Wei defeated Russian qualifier Veronika Kudermetova 6-2, 2-6, 6-2, and sixth-seeded Petra Martic ousted Chinese wild-card entry Liu Fangzhou 1-6, 6-0, 6-1.
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