Russian second seed Petrova was facing Pironkova in a straight shoot-out to see who would avoid the winner of Group Serdika, former world number one Caroline Wozniacki.
Petrova will play fourth seed Roberta Vinci of Italy for a place in the final.
"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.
"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."
Bulgarian wildcard Pironkova, the lowest-ranked player in the tournament at 44 on the WTA list, took the first set but was then blown away by the world number 13 and Russian number two.
The opener was a tight, cagey affair with Pironkova taking it as Petrova double-faulted twice on key points despite otherwise serving more effectively.
But Petrova fought back in style, dominating the second set and taking the third by virtue of an extra break as she won through in two hours and nine minutes.
Jie Zheng of China became the latest player to withdraw from the tournament, pulling out of her Group Sredets match against Sofie Arvidsson.
Sweden’s Arvidsson, herself a replacement after Maria Kirilenko withdrew with a respiratory illness, was 5-1 up in the first set when the Chinese number two quit.
Neither had a chance of qualifying for the semi-finals, with Petrova and Pironkova already through after Kirilenko fell ill.
With Wozniacki and Vinci already confirmed in the semis on Thursday, Su-We Hiseh's 6-1 0-6 6-4 win over Daniela Hantuchova was a dead rubber.
Wozniacki will return to the top 10 if she beats Pironkova in their last-four clash.
2-Nadia Petrova (Russia) beat 8-Tsvetana Pironkova (Bulgaria) 5-7 6-1 6-3
9-Sofia Arvidsson (Sweden) beat 6-Zheng Jie (China) 5-1 (Zheng J retired)
5-Su-We Hiseh (Taiwan) beat Daniela 7-Daniela Hantuchova (Slovakia) 6-1 0-6 6-4