Elena Rybakina maintained her undefeated record at the WTA Finals with a 6-4, 6-4 victory over Russian alternate Ekaterina Alexandrova in Riyadh on Wednesday.
It was a rematch of the Ningbo final from three weeks ago, which saw Rybakina capture her second title of the season.
The Kazakh world number six extended her current winning streak to nine of her last nine matches contested, and will face one of Aryna Sabalenka, Jessica Pegula or Coco Gauff in Friday's semi-finals.
"Ekaterina is always a tough opponent and has a big serve. I'm happy I was able to win in straight sets. Each win gives you confidence and I'm pretty happy that for now my last matches have been great," said Rybakina, who was playing with taping on her shoulder.
Rybakina entered the match with Alexandrova having already secured her place in the semi-finals as the winner of the Serena Williams Group, while Iga Swiatek and Amanda Anisimova will battle it out for the other ticket to the final four.
Alexandrova was brought in as a replacement for American Madison Keys, who withdrew ahead of the match due to a viral illness.
The Australian Open champion suffered two defeats in round-robin play this week in Riyadh, to Poland's Swiatek and compatriot Anisimova, and had no chance of advancing to the knockout stage.
At the end of her match against Anisimova on Monday, Keys wouldn't shake her opponent's hand, indicating she was feeling unwell and didn't want to pass on the virus.
"I'm really disappointed to not be feeling my best and ultimately having to withdraw from the tournament. It's a huge accomplishment to make it this far and I'm very proud of myself for doing that and having a great year. I hope to be back next year," said Keys in a statement.
Alexandrova is the second alternate in Riyadh. The first alternate, Mirra Andreeva, who is already competing in the doubles at this year's WTA Finals, elected not to sign in due to not being fit to play on Wednesday.
Andreeva has a doubles match on Thursday, alongside her partner Diana Shnaider, and might still be called in for duty as an alternate if any of the four singles players is unable to play.
Thursday will also see the conclusion of the Stefanie Graf Group, with Belarusian top seed Sabalenka taking on Gauff of the USA and America's Pegula scheduled to play Jasmine Paolini, although the Italian has also reported feeling unwell.
