Curiously, the two had never played each other before, despite years on the tour together - one could tell in the first two sets of the match, with neither player really settling into any consistent momentum.
Vesnina was the steadier of the two early on, going up a set and a break, 62 32, seemingly on the way to a straight set victory. But Pironkova began hitting the mark on her backhand and clawed her way back into it, winning four of the next five games to sneak out the 51-minute second set, 6-4.
Vesnina steadied herself again from there, though, winning 15 of the first 18 points of the third set and eventually taking the set at love for a 62 46 60 victory. The No.5-seeded Russian finished off the two-hour, 13-minute battle with a massive forehand winner the No.8-seeded Bulgarian couldn't retrieve.
"Today I felt in the tough moments the crowd was really behind Tsvetana, they were really trying to cheer for her," Vesnina said. "For me, I knew I just had to not think about that. I knew I just had to focus on myself, on my game, on my game plan - and it was a pleasure to play this match today."
Friday's wave of round robin matches will determine who moves through to the semifinals, with Ivanovic taking on Vesnina and Stosur taking on Pironkova. Ivanovic and Vesnina are up first.
"I'm so happy I can still make the semifinals," Vesnina said. "It's going to be a great experience for me to play against Ana tomorrow. She's a great player and a great person, and I'm going to go out there and try to win this match. I'm just happy I was able to qualify for this tournament, so to qualify for the semifinals would be amazing. I'm just enjoying my time here and I'll try to do my best tomorrow."