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.
Konta moved up to eighth in the standings following the withdrawal of world number two Serena Williams.
BBC tennis correspondent Russell Fuller said she has arrived in Singapore, and appears fit to play.
Kuznetsova is the only player who can catch her, as ninth-placed Carla Suarez Navarro retired with a wrist injury after round two of the Kremlin Cup.
Meanwhile, Angelique Kerber will end the year as the world number one.
The 28-year-old German, who won the Australian and US Open this year, succeeds American Williams, who held the year-end number one ranking for the past three years.
|Road to Singapore (470 points awarded for Moscow title)|
|1. Angelique Kerber (Germany) - Qualified|
|2. Agnieszka Radwanska (Poland) - Qualified|
|3. Simona Halep (Romania) - Qualified|
|4. Karolina Pliskova (Czech Republic) - Qualified|
|5. Garbine Muguruza (Spain) - Qualified|
|6. Madison Keys (USA) - Qualified|
|7. Dominika Cibulkova (Slovakia) - Qualified|
|8. Johanna Konta (Great Britain) - 3,455 points|
|9. Carla Suarez Navarro (Spain) - 3,170 points|
|10. Svetlana Kuznetsova (Russia) - 3,080 points|