Czech qualifier Lucie Hradecka on Friday stunned fifth seed Samantha Stosur beating the Australian in two tight tiebreakers in the quarter-finals of the Madrid Masters tennis event in Spain. Hradecka hit 19 aces in her 7-6 (10-8) 7-6 (8-6) upset of the US Open champion. On the men’s side, world No.1 and defending champion Novak Djokovic was also knocked out in the quarter-finals, losing 7-6 (7-2) 6-3 to Serb compatriot and seventh seed Janko Tipsarevic. Djokovic’s defeat came a day after world No.2 Rafael Nadal was defeated by fellow Spaniard Fernando Verdasco in the third round. In other quarter-final action, Argentina’s Juan Martin del Potro hit 10 aces to beat Alexandr Dolgopolov of Ukraine 6-3 6-4 and set up a semi-final with Tomas Berdych. The sixth-seeded Berdych defeated Verdasco 6-1 6-2. The Spaniard made 24 unforced errors and showed little of the flare on display on Thursday when he ousted Nadal. Roger Federer will play David Ferrer later on Friday. Hradecka will face Serena Williams for a place in the final after the American thrashed Maria Sharapova to reach the semis. Williams, the ninth seed, cruised to a 6-1 6-3 win over second-seeded Sharapova, the recent Stuttgart champion, hitting 11 aces, 28 winners and enjoying five breaks of serve in 80 minutes. Williams said while the blue clay courts remain problematic, the issue was not enough to make her consider skipping the event in 2012 as Nadal and Djokovic have threatened to do. Top seed Victoria Azarenka saw off China’s Li Na with a battling 3-6 6-3 6-3 win to also reach the semi-finals. The Belarusian world No.1 will play Agnieszka Radwanska, the Polish fourth seed, who beat American qualifier Varvara Lepchenko 6-4 6-4. Azarenka, January’s Australian Open champion, levelled her head-to-head record against French Open champion Li at 4-4 with the victory. Eighth seed Li was playing in her second clay quarter-final after Stuttgart. Azarenka said that after losing the opening set against Li, she still has no idea how she was able to turn the match around for victory in just under two hours. “She played unbelievable in the first set,” said the top seed. “I knew I had to do something. I don’t know if it was confidence or self belief or what. I don’t know what happened but I managed. I started to get into the match and build up my game.”