Aryna Sabalenka has been as high as number two in the WTA rankings, but is yet to reach a Grand Slam finalGreece's Maria Sakkari remains unbeaten at the WTA Finals as she defeated Tunisia's Ons Jabeur, a result which saw Belarusian Aryna Sabalenka's join her in the last four.
Sakkari won 6-2 6-3 in Texas and will face France's Caroline Garcia or Russian Daria Kasatkina in the semis.
Sabalenka, who earlier beat American Jessica Pegula 6-3 7-5, finished as group runner-up.
The seventh seed will now face world number one Iga Swiatek.
The season-ending event sees the world's eight best players this season split into two groups of four.
After the round-robin matches, the top two in each group move through to Sunday's semi-finals.
Sixth seed Garcia and eighth seed Kasatkina will play their final round-robin match on Saturday for a place in the semi-finals.
Poland's Swiatek has already won that group.
Jabeur had needed to win in straight sets to reach the knockout stage, but fifth seed Sakkari wrapped up victory in 69 minutes.
"I'm trusting myself, I'm just fearless on the court and I am enjoying myself here," said Sakkari.
"There is no secret behind how I am playing but I think it was about time just to start playing good again and I am very happy that it's actually happening here."
The WTA Finals is taking place in Fort Worth, Texas, after the event was moved from usual host city Shenzhen, in China, for the second successive year.
In December last year, the women's governing body announced the immediate suspension of all tournaments in China amid concern for Chinese tennis player Peng Shuai.
Peng, 36, disappeared from public view for three weeks after accusing a top Chinese official of sexual assault.
