Svetlana Kuznetsova is two victories away from denying Johanna Konta the last available WTA Finals spot after reaching the Kremlin Cup semi-finals.
Konta is in line to take the eighth and final place at the season-ending event.
The Briton, 25, is not competing this week as she recovers from injury and Kuznetsova can overtake her, but only if she wins the Kremlin Cup.
The 31-year-old Russian beat Hungary's Timea Babos 7-5 6-7 (2-7) 6-2 in the quarter-finals on Thursday.
Kuznetsova, the defending champion, will play Ukraine's Elina Svitolina in the semi-finals on Friday.
The WTA Finals begin on Sunday in Singapore.