All four semi-final places at the Qatar Airways Tournament of Champions were decided with one day of round robin matches still to go in Sofia.
Caroline Wozniacki qualified as the winner of the Serdika Group after extending her perfect run at the tournament with a 3-6 7-6 (7-4) 6-4 victory over Daniela Hantuchova.
The result ensures the Dane finishes top of the group while Italy's Roberta Vinci will join her in the last four after thrashing Hsieh Su-wei 6-1 6-2.
Hsieh and Hantuchova have both lost two matches so far, rendering their clash on Friday a dead rubber.
In the Sredets Group, Nadia Petrova qualified for the semi-finals with a 3-6 7-6 (7-4) 6-3 win over Maria Kirilenko, whose subsequent withdrawal from the tournament handed a place in the last four to home favourite Tsvetana Pironkova.
Petrova came out on top after a hard-fought battle with fellow Russian Kirilenko which lasted two hours and 48 minutes.
Kirilenko was still well placed to reach the semi-finals but pulled out with a respiratory illness, meaning Pironkova qualifies as she holds the head-to-head tie-breaker over Zheng Jie.
Friday's three matches are not completely insignificant, however, as Petrova and Pironkova will face-off to determine who finishes top of the group, with the loser facing Wozniacki in the semi-finals.