Serbia's Novak Djokovic faces the first real test of his US Open defence when he faces Argentina's Juan Martin Del Potro in the quarter-finals at Flushing Meadows on Friday (NZT).

Djokovic could hardly have had an easier time getting through to the last eight, winning each of his first three matches in straight sets, before his fourth round opponent Stanislas Wawrinka retired early in the third set.

But the world number two knows the easy part is over. Del Potro, U.S. Open champion in 2009, is also in great touch and presents a formidable challenge.

"Del Potro is a quality player, somebody that when he's physically fit he can challenge anybody on any surface," Djokovic said.

"It's quarter-finals. There is no really clear favourite anymore. I have to try to play my best."

The pair have played each other seven times before, with the Serb holding a 5-2 lead that includes victory in the Cincinnati Masters semi-finals last month.

The winner will advance to Saturday's semi-finals against either David Ferrer or Serbia's Janko Tipsarevic, who meet in Thursday's first quarter-final.

Ferrer was promoted to fourth seed after the withdrawal of his Spanish Davis Cup team mate Rafa Nadal and has eased his way through the draw with a minimum of fuss.

Tipsarevic, making his second successive appearance in the quarter-finals at Flushing Meadows, said he expected a torrid match.

"He is definitely a bulldog, I will tell you that," said Tipsarevic. " He runs down balls which maybe only Rafa can make from all the guys on tour.

"I have heard commentators on TV describing him as one of the guys who is able to put his body through so much during the match in terms of suffering and pain and doing whatever it takes to win, and that's, I guess, one of his biggest strengths."

- Reuters