Andy Murray
Andy Murray is seeking to triumph at Wimbledon for the second time in his careerAFP

Andy Murray is through to the quarter-finals at Wimbledon following a hard-fought four-set victory over Ivo Karlovic on centre court.

The 2013 champion, who battled back from a concerning mid-match wobble to beat Andreas Seppi in round three, coped well with his towering opponent's famously brutal serve and eventually claimed his seventh set point to take the opener on a tiebreak (9-7).

Murray then crucially broke Karlovic in the very first game of the second set and although the Croatian later saved two further break points down 3-4, he could not register his own breakthrough.

The British number one hurled his replacement racket to the court in sheer frustration after dropping the third set, but refocused to take the contest 7-6, 6-4, 5-7, 6-4.

"I came up with some good lobs and good passing shots," Murray said after the match. "It was a very difficult match, mentally tiring. It's not physically demanding, it's quite tiring because you need to be focused for long periods and wait for your chances to come. I felt like I passed well, when he missed his first serve I returned the second serve well."

Murray moves on to face Vasek Pospisil next. The Canadian, who outlasted James Ward over five sets on 4 July, went the full distance once more earlier on but successfully erased an early deficit to knock out Viktor Troicki 4-6, 6-7, 6-4, 6-3, 6-3.

On his clash with Pospisil, Murray added: "He obviously had a very close one with James Ward the other day and came back from two sets to love today so he may be a little bit tired but also very confident. He'll be feeling good about his game. He has a game that suits the grass."

Earlier in the day, Serena Williams followed up her entertaining win over Heather Watson in round three by beating sister Venus for the 15th time in her professional career.

In the 26th meeting between the siblings, both players broke serve in the first set but Serena eventually held her nerve to progress 6-4, 6-3 in 68 minutes.

Afterwards, the 20-time grand slam winner told the BBC: "It's never easy to play someone you love and care about, but you just have to stay focused. It's hard to get excited about beating someone you root for all the time, but that's the nature of the competition.

"I served well today, and that helped me to come through. It doesn't get any easier playing against Venus and I was thinking 'I'm 33 and she's 35 I just wondered how many more times we will get to do this, especially at Wimbledon?'

"I thought back to when we were kids dreaming of moments like this, and it feels a little surreal."

Serena will now play former world number one Victoria Azarenka in the last eight following her 6-2, 6-3 victory over Swiss Belinda Bencic.