Sofia - Flavia Pennetta beat Carla Suarez Navarro 6-4, 6-2 to set up a championship clash with Andrea Petkovic, who saw off Garbine Muguruza 6-1, 6-4, in the final of the WTA Tournament of Champions on Saturday.
Pennetta is through to the 24th WTA final of her career - she stands at 10-13 in the first 23.
"It was really tough from the beginning. The first four or five games were so long and so tight today," third seed Pennetta said in Sofia
"We know each other really well so we know how to play each other, and we always have a good fight until the last point. I was just trying not to give her the same rhythm all the time - I knew I had to change it up, but also be aggressive when the right moments came.
"Sometimes I made mistakes, but it was the right way to play today."
Pennetta and fourth-seeded Petkovic are tied 2-2 in their head to head series but they haven't played on hardcourts in four and a half years - and never on indoor hardcourts.
Petkovic will be playing in her third WTA final of the year, having already won titles at Charleston and Bad Gastein.
Sunday's match will be the German's 10th WTA final.