Timea Bacsinszky said there is a fire burning inside her and she is surprising herself after breaking French hearts to reach the Roland Garros semi-finals for the second time by defeating Kristina Mladenovic.

The 30th seed played in the last four of the French Open two years ago and will get another opportunity to seal a spot in a first grand slam final when she plays surprise package Jelena Ostapenko on Thursday.

Bacsinszky and Ostapenko will both be celebrating their birthday on the day of the match, but only one of them will be have reason to cheer after they have done battle in Paris.

The Swiss world number 31 spoke of her huge desire to continue her magnificent run on the famous clay in the second major of the year.

"I just know there is this fire inside of me which is keeping burning, which is warmer and which helps me a lot," said Bacsinszky, who is about to turn 28.

"You have your favourite surface. You have your favourite moments in the past in some tournaments.

"It's different for everyone. Roger [Federer], it was Wimbledon, Stan [Wawrinka], I think as far as I knew, it was the French Open, but Australia was probably something which became more important than the French the first time.

"But then he still played very well many times here at the French because he's, as well, a clay-court player.

"At the beginning he was more a clay-court player, I would say, because we shared – his former coach is my coach now, so that's the only info I've got."

She added: "I think the magic has always been happening for me here, even the first time I came here as a little kid. I am still finding out why. It is the only grand slam I think I have never lost yet in the first round.

"It's the grand slam which is closer to us from the region - I'm from a French-speaking part of Switzerland, so we have been following it quite a lot."

Asked if she is surprised with her exploits, Bacsinszky said: "I amaze myself sometimes with the focus I can have on some important moments during the whole tournament."