US Open 2011: Novak Djokovic & Roger Federer reach last 16

Djokovic has won 60 matches and lost just two so far in 2011

World number one Novak Djokovic remains on track for a first US Open title after seeing off Nikolay Davydenko in the third round at Flushing Meadows.

The Serbian racked up his 60th win of an amazing season as he swept to a 6-3 6-4 6-2 victory in the night match.

He will next face 22nd seed Alexandr Dolgopolov, after the Ukrainian beat Ivo Karlovic 6-7 (4-7) 6-2 6-4 6-4.

Roger Federer advanced into the fourth round after overcoming a spirited challenge from 27th seed Marin Cilic.

Number three seed Federer got the crucial break at 4-4 in the third set as he defeated the Croat 6-3 4-6 6-4 6-2 to reach the last 16.

Federer, 30, next meets Argentine Juan Monaco, who fought his way past Germany's Tommy Haas 6-7 (3-7) 6-3 6-2 6-3.

"Coming in I knew it was going to be tricky and I'm happy that I was able to counter his pace and his good play," said the Swiss.

"It was a tricky match, especially at one set all. At the beginning of the third set I thought that was a key moment because he had momentum on his side.

"I was not returning and serving exactly the way I wanted, but I was able to turn it around and finished strong in the set. Then in the fourth things were a bit easier."

Djokovic was given the prestigious night session slot once again and, while not matching the heights of some of his performances this year, he still had too much for Davydenko.

The Russian, ranked 39th, managed to recover one break in the opening set but not a second, and after saving seven break points his resistance finally broke in game seven of the second set.

"It's the first test I had in this tournament and it was expected," said Djokovic.

"I was aware of his qualities - Davydenko is a great player. He's been around for many years. He knows how to play on this stadium and he knows how to play majors."

Ninth seed Tomas Berdych is out of the tournament after injury forced him to retire during the second set of his match against Janko Tipsarevic.

The Czech was 4-6 0-5 down when a shoulder injury made him quit, allowing Tipsarevic through to play Spain's Juan Carlos Ferrero for a place in the quarter-finals.

Ferrero also benefited from a retirement as compatriot Marcel Granollers pulled out when 6-1 4-3 down.

Granollers, who had an abdominal problem, became the 18th player to retire at this year's US Open through either illness or injury.

Berdych said he first felt his shoulder injury during his quarter-final win over Federer in Cincinnati two weeks ago, and it then forced him to retire in his semi-final against Djokovic.

"I already knew there was a problem. I had it in Cincinnati, so it was a case of just trying," he said.

"I was lucky to finish the match against Roger [Federer] and I felt it here again in the last round. So I didn't hit on Friday, I just got treatment and tried to relax it."

American eighth seed Mardy Fish beat South African Kevin Anderson 6-4, 7-6 (7-4) 7-6 (7-3) and will now meet 11th seed Jo-Wilfried Tsonga, after the Frenchman defeated Spain's Fernando Verdasco 6-3 7-5 6-4.