The Swiss world number one took a while to warm up as clouds gathered over Arthur AsheStadium but once he had the first set in the bag, Federer wasted little time putting Robredo into the shade and secured his ninth straight win over the Spaniard.

Federer, winner of a record 15 Grand Slam titles, sealed victory with his ninth ace in one hour 48 minutes.

Federer next plays Sweden's Robin Soderling, whom the Swiss beat en route to the French Open and Wimbledon titles this year.

Roland Garros runner-up Soderling became the first man to reach the quarter-finals of the US Open when his eighth-seeded opponentNikolay Davydenko retired with a leg injury.

Russian Davydenko hobbled around court with his left leg heavily strapped but after losing the third set, he shook his head and informed the umpire he could not continue.The retirement would also have been a relief for Soderling, who had suffered a dizzy spell during the 7-5 3-6 6-2 win.

Fourth seed Novak Djokovic moved up a gear as he beat Czech 15th seed Radek Stepanek 6-1 6-3 6-3 to reach the quarter-finals.

The Serb, runner-up at Flushing Meadows in 2007, was never tested as he raced to victory in one hour, 46 minutes to set up a clash with 10th seed Fernando Verdasco of Spain.

Stepanek was woeful in the first set and Djokovic took it in just 23 minutes before easing through the second.

The Czech put up more of a fight in the third set but a brilliant lob gave Djokovic the break in the sixth game and he marched into the last eight for the third straight year.

Djokovic will next face Fernando Verdasco, who killed off American hopes in the men's singles, beating John Isner 4-6 6-4 6-4 6-4 to make it the worst home showing in the professional era.

The towering Isner, who knocked out American number one Andy Roddick in a five-set epic on Saturday, started well against 10th seed Verdasco but was eventually overpowered from the baseline by the classy Spaniard.

Isner banged down 13 aces to no avail against Verdasco who reached the semi-finals of the Australian Open this year.

Verdasco will face either fourth seed Novak Djokovic or 15th seed Radek Stepanek in the last eight.

Onions replaces Flintoff in Champions Trophy

LONDON: Fast bowler Graham Onions has replaced injured Andrew Flintoff in the England squad for the ICC Champions Trophy to be held in South Africa from September 22 to October 5.

The England and Wales Cricket Board said the ICC has conveyed to it that the Champions Trophy event technical committee has approved 26-year-old Onions as Flintoff's replacement.

Flintoff, who retired from Test cricket after England's recent Ashes success, was initially named in England's 15-member Champions Trophy squad but was later ruled out due to a knee injury.

Subsequently, Onions, who is yet to make his one-day international debut, has also been called up to join the England team for their ongoing ODI series against Australia.