Radwanska, who had already taken out No.3 seed Jelena Jankovic in the first round, was down a break no fewer than five times in the third set but managed to pull off yet another upset against the No.8-seeded Pironkova in the quarters, eventually closing it out in a third set breaker, 6-2, 3-6, 7-6(5).
Pironkova served for the match twice, at 5-4 and 6-5, and was even two points away at one point.
"It was my first time playing her and I think we both played great tennis," Radwanska said. "She was really fighting until the end, we both were, but that's tennis - only one player can win in the end."
Radwanska won the two-hour-and-16-minute marathon when one of her shots trickled over the net.
"The last point was so difficult to take, but I'm happy it went on her side!" she said.
It's been a rough few years for Radwanska. A former World No.29, she's had to battle her way back into the Top 100 after a few years in the tennis wilderness, and she's currently back up at No.99 in the world. It's looking very much up now, though - this is her first WTA semifinal since Guangzhou in 2012.
"I had shoulder surgery a few years ago, so this is me making my way back," Radwanska said.
"I'm very happy. I'm very pleased to be in the semifinals again."
And the Radwanska family in general has good memories here - Agnieszka Radwanska won here in 2008, and the sisters teamed up to win the doubles title here in 2007. Could 2015 bring another title?
"Me and my sister have a great history here," Urszula Radwanska said.
"I talk with her every day. She texts me and watches my matches. We have great memories here."
Rybarikova leads Radwanska in their head-to-head series, 3-1, and 2-0 in completed matches.
Earlier in the day, Lesia Tsurenko won an all-Ukrainian clash against Kateryna Bondarenko, a qualifier into the draw, fighting off four set points in a marathon first set tie-break - and then another four set points in the second set - to win a marathon two-hour-and-29-minute two-setter, 7-6(13), 7-5.
Tsurenko's next opponent will be Kirsten Flipkens, who outdid fellow veteran Francesca Schiavone in the night match, 6-2, 6-4. Flipkens has beaten Tsurenko in both of their previous meetings.
Hope everyone is supporting Ula in Istanbul ?????@WTA
? Aga Radwanska (@ARadwanska) July 24, 2015