“I struggled at the beginning to read his serve and I thought he played really well in the first set,” said Federer. “He deserved it more than I did.

“I had been missing a lot of serves but in the second set it got better. I don’t feel good just yet, but I have won three rounds on hard courts and in the second set I was more confident.

“I will need to play better than that in the next round,” said Federer although he acknowledged he will be the favourite to reach the semi-finals against his doubles partner Max Mirnyi.

The unseeded Belorussian, one of the few all-out serve-volleyers left in the game, followed his conquest of the fifth-seeded former world number one Lleyton Hewitt by beating the 11th seeded Paradorn Srichaphan 6-3 6-2.

Top seed Andre Agassi also booked his place in the last eight, although he was 4-1 down in the first set against the local wild card player Simon Larose before winning 6-4 6-2.

However once the 33-year-old American got the rhythm of his ground strokes going in variable weather, which changed rapidly from cloud and rain to fierce and stamina-sapping sunshine, he made the Canadian work too hard to stay in the match for long.

“I spend less time thinking about the (US) Open and more time on where my game is,” said Agassi.

“When I feel I’m about to hit my top level then I just look to maintain it, and to hit the gears as I need them come US Open time.

“Today was a big step forward for me. I got pretty comfortable in all aspects of my game.”

The other quarter-final in Agassi’s half of the draw will be between Argentinian David Nalbandian, last year’s Wimbledon runner-up, and Feliciano Lopez, the hard-hitting left-handed Spaniard.

This encounter will ensure an unseeded semi-finalist.

Lopez beat the Slovakian qualifier Karol Beck 6-4 7-6, while Nalbandian defeated American Vince Spadea 6-4 6-2.