Halep, who has won the tournament for the last two years, had looked imperious on her way to the last eight, dropping nine games in three matches including Wednesday’s victory over Pliskova’s twin sister Kristyna.
But her winning streak in the Spanish capital ended at 15 matches as her 26-year-old opponent continued her impressive form on the red clay having won the Stuttgart title last month.
It was sixth seed Pliskova’s first claycourt victory over Halep and it was the kind of authoritative performance that will turn a few heads with the French Open looming.
Pliskova will play fellow Czech Petra Kvitova in the semi-finals after the 10th seed beat Russian Daria Kasatkina 6-4 6-0.
Dutchwoman Kiki Bertens continued her impressive week as she backed up a third-round win over world number two Caroline Wozniacki to beat Maria Sharapova and advance to the last four.
Unseeded Bertens, who reached the French Open semis two years ago, recovered from losing the first set to win 4-6 6-2 6-3 and set up a semi-final with Caroline Garcia who eased past local hopeful Carla Suarez Navarro.
Bertens’ win over Wozniacki meant Halep will stay as world number one for at least another week, but the Romanian had no answer to Pliskova on centre court.
Pliskova broke for a 4-3 lead in the opening set and although Halep saved two set points serving at 3-5 Pliskova closed out the opener with a love hold.
Romanian Halep’s error-count was surprisingly high, and she found herself under pressure early in the second set as Pliskova pounced to break in the third game.
Halep could make little impression on Pliskova’s powerful serving and she buckled at 3-5, dropping serve again to bow out.
She quickly shrugged off the loss, however, saying she would now focus on Rome next week before turning her thoughts to another attempt to land a first grand slam title.
“I’m not stressing myself about the (ranking) points,” said Halep, who also has a lot of ranking points to defend in Rome having reached last year’s final. “I’m not stressing myself about this busy schedule.
“I just want to enjoy it. I’m looking forward already to start the next tournament. There is no pressure. I feel like it’s an opportunity to play my best tennis.
“Maybe I didn’t play my best game, but maybe she didn’t let me play my best game. She was really good today.”
