Andy Roddick has credited his quarter-final opponent Lleyton Hewitt with shaping the future of men's tennis.

The Australian rose to prominence during the era of the big servers and counter-punched his way to world number one, picking up grand slams titles at the US Open in 2001 and Wimbledon the following year.

Injuries have taken their toll since then and the 28-year-old is currently down at 56th in the world rankings after missing the latter part of last season to have hip surgery.

But he has rolled back the years this fortnight and will bid to reach his third semi-final at the All England Club when he meets Roddick on Court One later today.

The American said of his rival: "He probably set the bar for the way a lot of these guys can move now.

"Where (Jim) Courier probably took it in the mid-90s as far as working and making it physical, I think Lleyton maybe did that in the early part of this decade.

"I think a lot of what you're seeing now is because of these guys watching him and seeing how effective legs can be."

Roddick has won the last four matches between the pair, including in the third round at Queen's last month, but he will certainly not be underestimating the Australian.

"I have loads of respect for Lleyton, what he's been able to accomplish," said Roddick.

"Everyone knows he's certainly capable of playing very, very, very well on this surface. It will be a tough one.

"I've seen him play too much good tennis to have ever really let him drift too far from consciousness as far as the top guys go."

Hewitt has garnered nearly as much attention for his raucous fans as his impressive tennis, with a group of them warned by stewards during his fourth-set win over Radek Stepanek.

Roddick, though, insists he will not be intimidated.

"I've dealt with fans before," he continued. "I have no problem with the crowd getting rowdy, as long as the respect level is there."

Second seed Roger Federer will need to bring his best returning game to Centre Court when he opens proceedings against big-serving 22nd seed Ivo Karlovic.

The 6ft 10in Croatian has served 137 aces already this tournament, and Federer said: "It's maybe not the most fun match to go through but I like to beat this guy because he makes it hard on us to beat him.

"He's become an excellent player. Not only just his serve, he's got to have something more otherwise he wouldn't be ranked where he is and he wouldn't be beating all those good players. He's not to be underestimated."

Should Karlovic cause a huge upset in his first Wimbledon quarter-final, the crowd will be treated to his now customary victory jig.

"It will be the same dance," he added. "It just came because I was happy. After all of my matches here I was doing that."

Fourth seed Novak Djokovic will be looking to turn the tables on Tommy Haas after losing to the German in the final of the Halle warm-up tournament.

The Serbian has had a somewhat indifferent year but believes he is coming into form at just the right time.

"I played really well the last two, three matches," Djokovic said. "I'm quite confident."

Third seed Andy Murray, meanwhile, will hope to end the impressive run of wild-card entry Juan Carlos Ferrero.