A hurting Andy Murray survived a scare in his US Open first round match on Monday, holding on for a four-set victory over Robin Haase.

Murray defeated the 70th-ranked Dutchman 6-3 7-6 8-6) 1-6 7-5, but was at a loss to explain the painful cramp that struck without warning – and could have cost him the match.

“I could have easily lost that match,” said Murray, who thought he’d have been in real trouble in a fifth set.

“I was very close to losing the match.”

The Scot, who hasn’t reached a final since lifting the Wimbledon trophy in 2013, staggered through the third set, and twice rallied from a break down in the fourth to finally close it out after just over three hours.

The 27-year-old, who won the US Open in 2012, said he arrived at Flushing Meadows fitter than he has been for any Grand Slam of the year.

But the match against Haase left him looking for answers.

“I don’t know exactly why it happened today,” said Murray, who trains in the steamy heat of Miami and didn’t think the 30C temperatures on Louis Armstrong Stadium were excessive.

“The conditions in Miami were significantly hotter and more humid than it was out there today,” he said.

“And at the time it happened I wasn’t exhausted. It just happened.”

In the end, however, he was safely through, gaining the decisive break in the 11th game of the fourth set then saving three break points in the following game before closing out the match.

Australian Open winner Stan Wawrinka, seeded third, moved smoothly into the second round with a 6-2 7-6 (8-6) 7-6 (7-3) victory over Czech Jiri Vesely.

Milos Raonic, the big-serving Canadian who is among the young guns vying to end the Grand Slam hegemony of the game’s Big Four – Djokovic, Federer, Rafael Nadal and Murray – pelted Japanese qualifier Taro Daniel with 20 aces in a 6-3 6-2 7-6 (7-1) victory.