Petra Kvitova was nervous the first time she stepped onto Centre Court as defending champion a week ago, feeling the weight of the tradition and the expectations that come with that title.

She had overcome those nerves by the time she walked out again Tuesday to face Serena Williams in the quarterfinals, only to find a much more difficult opponent in a four-time Wimbledon winner who knows all about performing well on tennis' biggest stage.

Kvitova played one of her best matches of the tournament and still lost, overpowered 6-3, 7-5 by the hard-serving Williams, who reached her eighth Wimbledon semifinal.

"I think it was great match from both of us, and I think that she just served much better than me," Kvitova said. "I think there were some really important points that I could play better. I think that was the difference."

Kvitova has struggled following her first Grand Slam title at the All England Club last year, and has yet to reach a final in 2012. So coming in to Wimbledon as the woman to beat was a bit daunting.

"For me it was difficult (in the) first round when I went to Centre Court," Kvitova said. "That was tough for me to be strong mentally because I played badly before, so I didn't feel much better. Then I think after the first round was good, and I didn't thinking about that I'm defending champion. I just played every match ... and I was more relaxed."

It's just that Williams was simply better.

The 13-time Grand Slam winner hit 13 aces — including three in the last game — to end Kvitova's 11-match winning streak at Wimbledon. Her last loss here also came against the American, in the 2010 semifinals.

Williams seized control by breaking Kvitova for a 4-2 lead in the first set and then never gave her opponent a chance to get back in the match. She saved a set point when trailing 5-4 in the second with a hard serve down the middle that Kvitova returned into the net. Williams won 86 percent of the points on her first serve.

"I think that's why she's the great champion, because she knows what she needs to play in the important points," Kvitova said. "It's really tough to beat her."

Williams knows what's it like to be the player everyone is trying to beat at the All England Club. She has successfully defended the title twice — in 2003 and 2010 — before losing it the next year.

"It's definitely not easy. I think when I defended my first major I didn't win," Williams said. "But she did great. She held herself in ultimate class and played her heart out. I don't think I've ever seen her play so well."

Williams will next play Victoria Azarenka, who beat unseeded Tamira Paszek of Austria 6-3, 7-6 (4) despite failing twice to serve out the match.

In a match that was moved from Court 1 to the roof-covered Centre Court because of a series of rain delays, Azarenka was broken for the first time when leading 5-4 in the second set. She broke right back but then wasted a second opportunity to serve it out, before converting her second match point in the tiebreaker when Paszek sent a backhand long.

The other semifinal will feature Angelique Kerber against third-seeded Agnieszka Radwanska. Kerber beat fellow German Sabine Lisicki 6-3, 6-7 (7), 7-5 despite wasting three match points in the second set, while Radwanska reached her first Grand Slam semifinal by outlasting Maria Kirilenko 7-5, 4-6, 7-5 in a rain-affected match that saw several suspensions and was moved onto Centre Court from Court 1 for the last four games.

"I think today was for me, like, 40 hours," Radwanska said. "I was on and off all the time, waiting pretty much all day. So It wasn't easy for me, especially because of the conditions. It was raining and windy as well."

The rain caused disruptions for several of the men as well. Five fourth-round matches had been suspended in progress or postponed completely on Monday because of the persistent showers.

Jo-Wilfried Tsonga overcame an ailing back and a one-set deficit to beat American Mardy Fish 4-6, 7-6 (4), 6-4, 6-4. David Ferrer also advanced in the first match on Centre Court, where organizers again came under scrutiny for their use of the retractable roof. Their decision to open and then close it caused the start of Ferrer's match against Juan Martin del Potro to be delayed by about 40 minutes. Ferrer needed less than two hours to beat del Potro 6-3, 6-2, 6-3 and will face fourth-seeded Andy Murray.

Murray made the final eight for the fifth straight year by beating the 16th-seeded Marin Cilic 7-5, 6-2, 6-3, while Florian Mayer ousted Richard Gasquet of France 6-3, 6-1, 3-6, 6-2. American qualifier Brian Baker's remarkable run ended with a 6-1, 7-6 (4), 6-3 loss to 27th-seeded Philipp Kohlschreiber of Germany.

"We stopped, obviously, a lot," Murray said. "You're always a little bit edgy when you come out after a rain delay when you have the momentum with you. You stop, and then you come back out, and it feels a bit like you start from scratch again. I needed to serve well today and I got a lot of free points that helped me out."

The 126th-ranked Baker was trying to become the third American qualifier to reach the quarterfinals at the All England Club. He was playing in Wimbledon's main draw for the first time after being off the tour for more than five years because of a series of operations, one on his right elbow. He was ranked 458th at the start of 2012 and will next week rise inside the top 100.

"It's been an unbelievable run," Baker said. "Can't be too upset about that, even though as a competitor (I'm) definitely pretty frustrated right now. ... Hopefully I'll learn from it and have more opportunities."

In the other men's quarterfinals on Wednesday, defending champion Novak Djokovic takes on Mayer, six-time winner Roger Federer faces Mikhail Youzhny of Russia and Tsonga is up against Kohlschreiber.