Caroline Wozniacki crashed out of the Madrid Open on Thursday with a second sucessive defeat to Julia Georges of Germany.
Georges, ranked 27th, defeated the world number one in the final of the Porsche Grand Prix in Stuttgart last month.
And she followed up that upset win despite a mid-match slump, recovering to beat the Dane 6-4 1-6 6-3 in their third round clash.
Georges found herself 3-0 in the opening set but rallied to break twice in winning six of the next seven games.
Wozniacki gained two breaks of her own to level matters but the German got the decisive break in the eighth game of the decider before comprehensively serving out for the match to book her place in the quarter-finals.
"The second set she started to be more aggressive and I started to get tired, so I said to myself keep yourself together for the third set and try to fight for every point," said Georges.
"I beat her twice and twice in two weeks so I'm pretty proud of myself."
Anastasia Pavyluchenkova awaits in the last eight after the Russian saw off fifth seed Sam Stosur 7-6 6-3.
Victoria Azarenka continued her blistering form when she crushed Spanish wildcard Arantxa Parra Santonja 6-0 6-3 to set up a quarter-final against Czech Lucie Safarova or Jarmila Gajdosova of Australia.
The Belarussian fourth seed has dropped only five games in her three matches this week and is chasing a third title of the year following triumphs in Miami and Marbella.