LONDON, July 1 (Xinhua) -- Six-time champion Serena Williams suffered a shock defeat in the fourth round at the Wimbledon Championships here on Monday as men's top seed Novak Djokovic and second seed Andy Murray entered the quarterfinals without dropping a set.

Djokovic defeated Germany's Tommy Haas 6-1, 6-4, 7-6 (4) while Murray took two hours and 37 minutes to beat Mikhail Youzhny of Russia 6-4, 7-6, 6-1.

Germany's 23rd seed Sabine Lisicki produced another shock victory as she stunned world No. 1 Williams 6-2, 1-6, 6-4.

She has beaten the reigning French Open winner at Wimbledon the last four times she played here: 2010 against Francesca Schiavone, 2011 over Li Na, last year vs. Maria Sharapova and again beating Williams.

"I'm still shaking," Lisicki said in a post-match interview. "I'm just so happy."

Williams said her serve failed her in the third set.

"I felt that I was on the verge of winning," she said. "At that point I just was physically unable to hold serve. ... You have to be ready and willing to hold your serve. I wasn't willing or able, probably didn't even want to hold my serve today."

Djokovic and Haas, who is 35, had different missions when they came to the Center Court in the early evening. Djokovic simply wanted to maintain form and belief and reach his 17th Grand Slam quarterfinal while Haas was looking to become the oldest Wimbledon quarterfinalist since Dutchman Tom Okker in 1979.

But in little more than 70 minutes Djokovic held a two-set lead and fought off tough challenge in the third to seal the win.

"I think that I'm playing really, really good tennis at this moment," Djokovic said. "Maybe even better than back in 2011 when I won this tournament."

Djokovic will face the Czech Tomas Berdych in the quarterfinals. He holds a 13-2 lead head-to-head but lost to him at Wimbledon in 2010.

Djokovic said: "There is no reason I am concerned about my game. I am trying to combine offensive and defensive skills in all of my matches."

Murray, who pulled out of the French Open five weeks ago due to injury, dashed Youzhny's hops of entering his second consecutive Wimbledon quarterfinal.

"I just have to concentrate on the next match," said Murray. "Serena Williams lost today and she doesn't lose particularly often. Roger lost, Rafa lost - all these guys and girls are better than me and if they can lose, so can I."