(CNN) -

Andy Murray started the 2014 season off slowly, which might have been predicted given he had back surgery last fall. He picked up his game at the French Open only to suffer a bewildering loss to Grigor Dimitrov at Wimbledon as the defending champion.

Things almost got worse for Murray on the opening day of the U.S. Open, as he suffered from cramps throughout his body before seeing off Dutchman Robin Haase 6-3 7-6 1-6 7-5 on Monday.

Despite Murray being in distress, Haase couldn't serve out the fourth set and also squandered a break chance in the final game when he netted a fairly routine forehand volley.

For much of the last two sets, Murray struggled with his movement, didn't get pace on his serve and clutched at his body.

Haase was "incapable of taking his opportunity," Boris Becker, the coach of world No. 1 Novak Djokovic who was analyzing the match for Sky Sports, said. "Murray was presented to Haase on a silver platter in that fourth set but Robin didn't like it."

Murray, known for his endurance, said he began to cramp after an hour and 45 minutes.

That wasn't "normal," he added.

"And then I didn't really know what to do in that third set, because my quads were cramping and then I started to get it in my lats so when I was throwing the ball up to serve, and then in my forearms," Murray told Sky.

"I didn't know whether to conserve energy or to try and finish it in three sets, but then I got broken at the beginning of the set. I just tried to hang around. I was trying to play without using my legs much."

Murray -- taken to five sets by Haase in New York in 2011 -- was also left wondering why his body let him down.

"I need to work out why that happened today because it shouldn't happen after an hour and 45 minutes regardless of the temperature," he said. "It was hot but it wasn't particularly humid and we didn't play loads of long rallies in the first couple of sets, either."

If it was only down to cramps, Murray could very well make a complete recovery and the 2012 champion caught a break in the draw because his next opponent is Matthias Bachinger, not tricky Czech Radek Stepanek.

Bachinger upset Stepanek in straight sets.

In the women's draw, second-seed and French Open finalist Simona Halep survived a scare, too, eventually overcoming inexperienced American Danielle Rose Collins 6-7 6-1 6-2.

There were victories for fourth-seed Agnieszka Radwanska, sixth-seed Angelique Kerber and 19th-seed Venus Williams.

Williams, 34, beat the nearly 44-year-old Kimiko Date-Krumm 2-6 6-3 6-3.