In the end Stosur came through. The No.4-seeded Australian was lethal on her serve - she held all nine of her service games, winning 18 points in a row on her serve at one point - and broke three times in the match to beat the No.8-seeded Bulgarian, 61 64. She fired a big ace up the T on match point.
"I definitely got a lot tighter as the match went on," Stosur said. "I got off to a great start, then a tough service game trying to serve out the set. Thankfully I got through that, then it was a matter of holding serve from there. I got the break in the second set and managed to close it out, so I'm happy.
"It's been great to be able to carry on from the last three or four weeks. To come here, the last event, to finish the group on top, hopefully I can continue the momentum in the semifinals tomorrow."
And on her serving? "When I serve well, and serve at a high percentage, I tend to win a lot of points with it, so that always helps my game. It's been good so far - hopefully I can pick it up even more."
And so the final four is set - Halep, Ivanovic, Stosur and Pavlyuchenkova. Group Serdika saw Halep finish first and Pavlyuchenkova second; Group Sredets saw Stosur finish first and Ivanovic second.
The semifinal format pits first place from one group against second place from the other, and vice versa - so Saturday's semifinals are Halep against Ivanovic and Stosur against Pavlyuchenkova.
First up is the No.1-seeded Halep against the No.2-seeded Ivanovic. Ivanovic beat Halep in their only previous meeting, though it did come via retirement - Halep retired after a set with a back injury.
Next up is the No.4-seeded Stosur against the No.6-seeded Pavlyuchenkova. Stosur leads Pavlyuchenkova in the head-to-head, 2-1, though it has been two and a half years since they last played, and they have never played on indoor hardcourts - all three meetings were outdoors.