Nadia Petrova clinched top place in the Sredets Group after coming from a set down to beat Tsvetana Pironkova at the Qatar Airways Tournament of Champions.
The Russian dropped the opener in the second group match, but only lost four more games in the match after that as she ran out a 5-7 6-1 6-3 against the Bulgarian.
Petrova will now face Roberta Vinci in Saturday's semi-final, while top-seeded Caroline Wozniacki will play Pironkova in the other clash.
"Roberta is one of those tricky players - she uses a one-handed backhand and uses a lot of slice, which is unusual for the women's game and brings a lot of trouble for her opponents," Petrova said on the WTA's official website.
"But since we're playing indoors the conditions are faster, so if I'm serving well and putting pressure on her service games and being aggressive, I hope tomorrow's match will be in my favour."
In Group Serdika, Sofia Arvidsson, who replaced the injured Maria Kirilenko, secured victory without finishing the first set as Zheng Jie of China retired at 5-1.
The result mattered little as neither could qualify for the knockout stages of the tournament.
Taiwan's Hsieh Su-wei defeated Daniela Hantuchova 6-1 0-6 6-4 in the last match of the day to avenge her loss to the Slovak in the PTT Pattaya Open semi-finals in Thailand in February.