SYDNEY - Bulgarian qualifier Tsvetana Pironkova reached her first WTA final with a semi-final win over second seed Petra Kvitova at the Sydney International on Thursday.

The world number 107 defeated her second top-ten ranked opponent of the week in a 6-4 6-3 victory. Pironkova, 26, will play fifth seed Angelique Kerber in Friday's final after the German left-hander mastered American Madison Keys 6-4, 6-2. "It's definitely a milestone. This is something I've been dreaming about for a long time now and something that I've been missing, to go to the final or even have a title," Pironkova said.

"I have been here a few times before. I've been playing qualifying here and I actually never imagined getting to the final here." Pironkova ranked as high as 31 in the world in 2010 following her Wimbledon semi-final appearance against Vera Zvonareva, and lost to Kvitova in the Wimbledon quarter-finals the following year. The Bulgarian has now won seven matches to get to the Sydney final, and reached the semis after beating Italian third seed Sara Errani in straight sets.

"I've had some pains here and there. It's quite normal considering I've been playing for seven consecutive days now," Pironkova said. "But these are not serious injuries. These are normal things that happen to you after so many matches. I've had them before. I know it's nothing to worry about too much." Kvitova said Pironkova had proved a difficult opponent.

"She moved well on the court, every ball was in the court," the Czech world number six said. "It was tough for me to make a winner because she moved in the baseline very well. This was hard for me to have short rallies. "She served well and she was returning my serve pretty well." Kerber's powerful double-handed backhand was again on top form during her win over Keys. "I am trying to be more aggressive than some months ago and try to play my game plan from the first point, and it's working. So I think that's better than last year," Kerber said.

Meanwhile, World number five Juan Martin del Potro recovered from losing the second set to beat Radek Stepanek and book a place in the semi-finals of the Sydney International on Thursday.

The Argentine, who boasts a huge forehand, downed the Czech Davis Cup hero 6-4, 3-6, 6-3 to set up a last-four clash against Russia Dmitry Tursunov, who advanced with a 7-6 (7/5) 6-2 win against Uzbek Denis Istomin. In the night quarters, Australian Bernard Tomic stayed on track to defend his Sydney title with a 6-4, 6-3 win over Ukraine's Alexandr Dolgopolov in just over an hour. Tomic will now play another Ukrainian -- Sergiy Stakhovsky, who ended Australian Marinko Matosevic's run 6-4, 6-3.

Top seed del Potro, the 2009 US Open champion who has worked his way back up the rankings after wrist surgery, needed his big serve to get him home after Stepanek levelled the match. "Radek improved his game in the second set. I was positive, waiting for my chance, and I played a fantastic two points in the third game of the third set to break his serve," del Potro said. "Then I served OK. Just doing my job and I closed the match."

Del Potro, 25, broke in the 10th game to take the first set, but 35-year-old Stepanek, who has featured in the top 100 for the past 12 years, broke three times in the second set to square the match at one set apiece. With his big serve and trademark crosscourt forehand, del Potro took charge in the decider and broke twice to seal the win. "I like to play a couple of matches before a Grand Slam," said the Argentine. "Play in a good tournament like Sydney, it's a big opportunity for me and also train before a Grand Slam."

Del Potro has had tough workouts in his two matches in Sydney with his first against Frenchman Nicolas Mahut also going to three sets. "Dmitry is a difficult opponent for me. He plays really flat and good shots down the line, backhands and forehands," he said. "He's the past champion of this tournament. He likes these courts and the conditions." In six previous encounters Tomic had only managed one win - a five-set slog in the second round of the 2012 Australian Open - but this time he made light work of the quirky Ukrainian.

"He's a bit like me but different. Not a lot of the players like playing him. He's very difficult to play," Tomic said of Dolgopolov. "I knew as soon as I was playing him it was going to be a difficult match. I don't like playing him. I played the right tennis. I was very focused and very clear what I needed to do."