Elina Svitolina's place at the WTA Finals was sealed when Karolina Pliskova lost 6-1 6-2 to Vera Zvonareva in the second round of the Kremlin Cup on Wednesday.
Second seed Pliskova went down in straight sets against the Russian qualifier, putting her place in the season-ending tournament in jeopardy.
The Czech is now relying on Kiki Bertens failing to reach the semifinals in order to take the final remaining spot in Singapore.
Pliskova lasted just 73 minutes against two-time grand slam runner-up Zvonareva, who claimed her first win against a top-10 opponent in almost seven years.
World number six Svitolina, who is not in action on the WTA Tour this week, qualified for the Finals for the first time last season but failed to get beyond the round-robin stage.
Bertens received a first-round bye in Moscow and will have to overcome Aliaksandra Sasnovich and Daria Gavrilova or Johanna Konta in order to secure a berth in Singapore.
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