Kuznetsova, 31, beat Czech Pliskova 3-6 6-2 7-6 (8-6) but her semi-final spot was sealed by Agnieszka Radwanska's 7-6 (7-1) 6-3 win over Garbine Muguruza.
Defeat eliminated Spain's Muguruza at the group stage.
The winner of Polish defending champion Radwanska's final White Group match with Pliskova will also go through.
Both Kuznetsova and Pliskova served for the match in the third set, before Kuznetsova converted her fourth match point,
"I felt the support of the crowd and I couldn't let anyone down. I had to fight," she said.
On Monday, Kuznetsova also saved a match point in beating Radwanska, after cutting her own hair during a changeover because she said it was affecting her play.
She earned the eighth and final place at the Finals by winning the Kremlin Cup on Saturday.
The top two players from each group will qualify for the semi-finals.
In the Red Group, world number one Angelique Kerber has all but secured her place in the last four, while Madison Keys and Simona Halep remain in contention.
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Svetlana Kuznetsova became the first player to reach the last four of the WTA Finals following victory over Karolina Pliskova in Singapore.