Fourth seed Novak Djokovic struggled to fight through the first round of US Open by five sets in a hot day of 96 degrees of Fahrenheit on Tuesday.

Djokovic, runner-up here in 2007 and a semi-finalist in the last two years, losing to Roger Federer on all three occasions, faced strong challenge from close friend and Davis Cup teammate Viktor Troicki. But the Serb rallied back into the match and pulled off a 6-3, 3-6, 2-6, 7-5, 6-3 win in three hours and 40 minutes.

Djokovic was two sets to one down and faced a break point for 1- 4 in the fourth set, but he survived the first round exit scare to advance into second round against Philipp Petzschner of Germany for a place in the third round.

But the struggle once again exposed his vulnerability when playing in hot conditions with the temperatures on the Arther Ashe Centre Court soaring to a peak of 96 degrees Fahrenheit (35.5 degrees Celsius) in mid-afternoon.

"Give credit to Viktor. He was playing well and serving incredibly well," Djokovic said.

"Even if I would lose today it would have been deserved on his side. A couple of points changed the match and I was just fortunate to get through."

Seeds who failed to make it past the opening round were 16th seed Marcos Baghdatis, who went down 6-3, 2-6, 1-6, 6-4, 7-5 to French veteran Arnaud Clement, and 24th seed Ernests Gulbis, who fell 6-2, 7-6 (7/1), 6-4 to Jeremy Chardy of France.

American 19th seed Mardy Fish almost joined them before he got the better of Jan Hajek of the Czech Republic 6-0, 3-6, 4-6, 6-0, 6-1 in a roller-coaster of a match.

Spanish 23rd seed Feliciano Lopez also made it through defeating Santiago Giraldo of Colombia 6-4, 6-4, 6-4.

Djokovic had not lost in the first round of a Grand Slam tournament since the 2006 Australian Open, but he looked to heading that way when Troicki had a break point for a 4-1 lead in the fourth set.

The former Australian Open champion saved this with a big crosscourt forehand winner and he battled back to 2-3 down.

Djokovic then levelled at 3-3 as Troicki struggled with his serving, which had been top-class up until then.

That break seemed to energise Djokovic and unsettle Troicki, who suddenly started to look the more tired.

Games went with serve until 6-5 for Djokovic when three unforced errors from Troicki sent the tie into a decider.

It keeps Djokovic on track for another crack at Federer in New York in the semi-finals. Federer won his first-round tie on Monday evening, defeating Brian Dabul of Argentina in straight sets.

Baghdatis, one of the form players coming into New York, refused to blame the heat for his loss to Clement, instead saying that his batteries had already been at low going into the tournament.

"I was really heavy on court and not feeling very fit, very defensive and I didn't play very good tennis," he said.

"Maybe a bit tired. It's been a month and three weeks I've been out here (United States) and that's a lot of time. And it's also first round of a Grand Slam and the nerves are there also.