• Heat takes toll on Djokovic who wins in three sets
• Tempers rise with temperature at Flushing Meadows
• Sousa on seven-match losing streak before US Open

NEW YORK: Novak Djokovic sweated his way to a 6-3 6-4 6-3 fourth round win over Portugal's Joao Sousa as the heat returned to the US Open on Monday, setting up a potential quarter-final with second seed Roger Federer.

Djokovic improved his perfect record in US Open fourth round contests to 11-0, leaving it to Federer, who plays Australian John Millman in the late contest on the Arthur Ashe Stadium court, to give the fans a mouthwatering match-up.

A Federer versus Djokovic clash would provide the perfect contest to propel the US Open into the second week, with the two champions having claimed seven titles at Flushing Meadows between them.

"The matches I've played against him (Federer) have shaped me into the player that I am today," said Djokovic, who beat the Swiss at the Cincinnati final in the lead up to the US Open.

Sousa arrived at the season's final Grand Slam in dreadful form, having lost his previous seven matches, but suddenly found his game on the hardcourts to knock out 12th seed Pablo Carreno Busta and number 17 seed Lucas Pouille en route to the last 16.

The first Portuguese player to reach the fourth round at a Grand Slam, the 68th-ranked Sousa never threatened to progress any further as his magical U.S. Open run came to an end.

The biggest concern for sixth-seeded Djokovic might well have been the conditions as the heat returned to New York after a few days of relief.

DJOKOVIC SUFFERS

Through his opening round matches the Serb and his performances suffered under a blazing sun and, with temperatures once again nudging into the 90s Fahrenheit (32 Celsius), the extreme conditions again took a toll on 31-year-old.

He went off court two sets and 2-1 up for a medical evaluation.

"I am not 21 anymore, that was 10 years ago. I still don't feel old but at the same time there is a little biological clock that is not really working in your favour," said Djokovic.

"Heat was the adversity today. I've experienced that in the first round. It's tough.

"It's not easy to play in these kind of conditions. At the same time, you can't do anything but try to be tough and survive, you know, find a way to win."

The 13-times Grand Slam champion made easy work of the opening set, breaking Sousa twice -- the second time to close out the set.

Things got tougher in the second as a sluggish Djokovic laboured before again clinching it with a break.

As temperatures rose so did tempers, with both players engaging in animated arguments with chair umpire James Keothavong over points.

Djokovic, however, was able to keep his cool just long enough to finish off Sousa with another late break.