Petra Kvitova's title defence at the Mutua Madrid Open did not last long.
The Wimbledon champion was beaten 6-4 6-3 by Czech compatriot Lucie Hradecka in the Spanish captial on Wednesday.
Kvitova, seeded third, was broken five times by her opponent, who needed to qualify for the main draw.
Hradecka said knowing Kvitova's game had helped her claim what was the biggest win of her career.
She told the WTA's official website: "I knew if I played strong and concentrated on my serve I had a chance to win.
"We played at the end of last year in some club matches and I won, and also we've played a lot of sparring games from Fed Cup. We know each other's games pretty well."
In other second-round matches, fourth seed Agnieszka Radwanska faced no such problems on her way to a 6-0 6-1 rout of Sara Errani, while Roberta Vinci looked equally ruthless in despatching Yanina Wickmayer 6-1 6-2.
Maria Kirilenko, the 16th seed, was upset though, beaten 6-4 6-4 by Ekaterina Makarova.
In the pick of the day's third-round action, world number one Victoria Azarenka saw off one of her predecessors, Ana Ivanovic, 6-4 6-4.
Eighth seed Na Li, the reigning French Open champion, saw off Angelique Kerber 7-5 6-4 while Samantha Stosur, seeded fifth, was a 6-3 6-2 winner over Petra Cetkovska.