Svetlana Kuznetsova is just two victories away from sealing the last place in the WTA Finals after fending off the challenge of Timea Babos to reach the semi-finals of the Kremlin Cup.
Russian Kuznetsova needs to defend her title in Moscow this weekend to claim the eighth and final spot in the season-ending tournament in Singapore next week.
Johanna Konta will qualify if Kuznetsova falls short in her homeland, but Babos failed to do the Brit a favour as the top seed won 7-5 6-7 (2-7) 6-2 to ensure she will face Elina Svitolina in the last four.
Kuznetsova broke Babos six times in a match which swung this way and then the other as she strives to prolong her season after being given a wildcard entry for the competition.
Babos was unable to seize the initiative after bossing a tie-break to level the match, so Kuznetsova remains on course to play in her first WTA Finals since 2009.
Fourth seed Svitolina took only an hour to oust lucky loser Ana Konjuh 6-1 6-1 in a one-sided quarter-final.
Konjuh, ranked 58, endured service woes as she double-faulted seven times and was broken on five occasions to bow out.
The 2014 champion Anastasia Pavlyuchenkova will not regain the title after Daria Gavrilova consigned the seventh seed to a 7-5 4-6 6-3 defeat, setting up a meeting with Julia Gorges after she dumped out eighth seed Daria Kasatkina.