The previous two matches between the pair had both gone Pironkova's way and she looked poised to extend this streak when she took the opening set.
However, in a match that hinged on just a handful of points, it was Hercog who kept her cool when it mattered over the following two sets, overcoming an inopportune rain delay and a 4-2 deficit in the decider to triumph, 36 75 64.
"I didn't have a great record against her so I'm really happy to finally beat her," Hercog said. "It was not an easy match and I had to fight for every point as there weren't many gifts from her."
Meeting Hercog for a place in the final will be Mona Barthel, who saw off home favorite Sofia Arvidsson, 63 64. Despite arriving in Sweden on a run of five straight defeats, the No.7 seed has looked in fine form this week and is one of only two players yet to drop a set.
The other is last year's runner-up Johanna Larsson. After Arvidsson's exit, Larsson is the last remaining Swede in the draw and she gave the crowd plenty the cheer about when she stunned No.3 seed Anabel Medina Garrigues, 64 64, in their rain delayed second round match-up.
A few hours later she was back out on court again, albeit just for a few minutes - a viral illness forced her quarterfinal opponent, Klara Zakopalova, to retire after just a handful of games.
"I'm feeling pretty good after a pretty long day," Larsson said. "It's nice to be in the semis so I'm really happy about that. I thought I played really well in the first match, especially on the forehand, and my game is just where I want it to be right now."
Standing between Larsson and a second consecutive final will be Mathilde Johansson. Born just a few hundred kilometers up the road in Gothenburg, Johansson - who now resides in and represents France - has certainly looked at home in Bastad, producing some of her best tennis of the year to reach the last four.
Like Larsson, Johansson was a victim of Thursday's torrential rain but showed no adverse signs from playing back-to-back matches, defeating Shahar Peer, 62 36 62, and then upsetting No.5 seed Anastasia Pavlyuchenkova, 64 60.
"It was a really, really long day! Raining, then stopping, then playing and then raining - it was pretty tough," Johansson said. "The morning match was really tough but in the afternoon it was a bit easier as I think I was more used to the conditions than Anastasia so the bad weather kind of helped me out!"