Top seed Justine Henin will face qualifier Timea Bacsinszky in the Antwerp tournament semi-finals after the Swiss teenager upset third seed Daniela Hantuchova on Friday.
Bacsinszky went through when third-seeded Slovak Hantuchova retired due to breathing problems with the 18-year-old Swiss leading their quarter-final 6-2, 4-6, 4-1.
World number one and local favourite Henin later won her match against Russian Alisa Kleybanova, ranked 106 in the world and like Bacsinszky a qualifier, 6-4, 6-3.
There was another upset when Karin Knapp beat fourth-seeded Swiss Patty Schnyder 6-2, 6-7, 7-6. The Italian now faces ninth seed Li Na of China who beat Swede Sofia Arvidsson 7-5, 6-4.
"This is indeed my week," said an elated Bacsinszky. "I survived three qualifying matches and saved a match point in the first round. That just boosted my confidence.
"Today, I played a solid match against the world number eight... Hantuchova's retirement at 4-1 (down) in the third set does not matter much to me. I believe my victory was legitimate and amply deserved," she told reporters.
"A semi-final against Justine Henin will of course be magnificent but I'll go into the match with the same strategy -- I concentrate on my game and try to block out the context."
It took Henin until the ninth game of the opening set to make the breakthrough against Kleybanova, her second break point giving her a 5-4 lead.
The Russian seemed likely to pull level again immediately when she had three break points but the Belgian defended them before taking the set with a forehand volley.
Henin went 5-2 up in the second but lost her serve for the second time in the set before taking the match with another forehand on her opponent's serve after one hour and 45 minutes.
Schnyder made a steady start and was level at 2-2 in the opening set before fading badly. She surrendered a 5-2 lead in the second set before sweeping through the tiebreak 7-1 but Knapp easily won the deciding third-set tiebreak 7-2.