Kim Clijsters' 17-match winning streak was ended by Russia's Svetlana Kuznetsova in the semi-finals of the J&S Cup in Warsaw on Saturday.
Belgium's Clijsters, who had won consecutive tournaments at Indian Wells and Miami, lost 2-6 6-2 6-3.
The seventh seed led 3-1 in the third set but missed a host of chances to allow Kuznetsova to take the match.
In the final, second seed Kuznetsova will face Justine Henin-Hardenne, who beat Serbia's Ana Ivanovic 6-4 7-5.
Kuznetsova was in buoyant mood after reaching her first final of the season.
"This is my best tennis this year," she said. "I'm really excited about it because I didn't start the season how I would like.
"I can't say I played horrible but I couldn't get the results I wanted.
"Today, I was tense in the first set but in the second set I started to be more relaxed and to play my game and not expect mistakes from her.
"I started to go for my forehand and just enjoy the tennis."
Clijsters was upbeat despite her defeat, saying: "This was the first match on clay where I've felt comfortable.
"I can take a lot of positive things out of it and, although I lost, I'm the most satisfied with this match out of the whole week."
Henin-Hardenne paid tribute to Ivanovic after booking her place in Sunday's final.
"It was a good fight and maybe I should have won the match earlier but she kept fighting," said the Belgian.
"I was playing much better in the second set but couldn't finish the points."
Ivanovic blamed poor footwork for her defeat.
"I didn't move that well today," she said. "I made a lot of mis-hits on both backhand and forehand.
"I should have got more into the court and put her under pressure. But it was a great experience and I think I had my chances in both sets."