Andy Murray
Andy Murray hits a return to Robin Haase during their match first round match at the US Open.Reuters

World number one Novak Djokovic faced no difficulties in the opening round of the US Open, as the Serbian thrashed his opponent in straight sets, while Brit Andy Murray had to battle cramps and Dutchman Robin Haase to advance on Monday.

Murray defeated Haase 6-3, 7-6 (6), 1-6, 7-5 at the Louis Armstrong Stadium in just over three hours. The two-time grand slam champion won the first set easily and prevailed over the Dutchman in the second set tie-breaker to build a 2-0 advantage.

The former US Open champion, though, soon struggled with cramps on an afternoon, which was hot but not particularly humid. As Murray started to struggle with his forehand and serve, Haase made most of the opportunity and clinched the third set quite comfortably.

Murray's woes continued in the fourth set and he faced break points twice, but the unforced errors committed by Haase came to his rescue and the Brit managed to reach the second round.

"I don't know what it was. I was cramping after one hour and 45 minutes," Murray pointed out "I didn't know what to do in the third set. I started to get it in my laterals and forearms when I was serving. I didn't know whether to conserve energy or try to finish it.

"I tried to hang around and tried to play without using my legs much. I managed to get through. These slams are physically challenging, but I need to work out why it happened. It shouldn't have happened, regardless of the temperature."

Meanwhile, top seed Djokovic recovered from his disappointing hard court season and dispatched Diego Schwartzman of Argentina 6-1, 6-2, 6-4 at the Arthur Ashe Stadium to book his second round clash with Frenchman Paul-Henri Mathieu.

The seven-time major winner easily won the first two sets against the Argentine but faced stiff resistance in the third. Schwartzman broke Djokovic's serve in the sixth game but the Belgrade-native got the break back in the following game and wrapped up the encounter in just 97 minutes.

"I thought I hit the ball very well throughout the whole match," Djokovic said. "The last three weeks actually, Toronto and Cincinnati were a bit tough emotionally for me. I wasn't finding my game. I wasn't able to feel comfortable on the court. But now it's different. I feel emotionally recharged and ready to go."

Milos Raonic also eased into the second round with a 6-3, 6-2, 7-6 (1) victory over qualifier Taro Daniel. Daniel had no answers to Raonic's power serves, as the Canadian fired 20 aces and booked his second round clash with Peter Gojowczyk of Germany.

Third seed Stanislas Wawrinka registered a 6-2, 7-6 (6), 7-6 (3) victory over Jiri Vesely of Czech Republic. Jo-Wilfried Tsonga also moved into the second round with a hard-earned 6-3, 4-6, 7-6 (2), 6-1 win over Juan Monaco.

Australian prodigy Nick Kyrgios, Fernando Verdasco, Tommy Robredo, Jeremy Chardy and Andreas Seppi also won their respective opening round matches.