A hurting Andy Murray held on for a four-set victory over Robin Haase on Monday to reach the second round of the US Open.

In steamy 30-degree heat on the Louis Armstrong Stadium, Murray defeated the 70th-ranked Dutchman 6-3 7-6 (8-6) 1-6 7-5.

The Scot, who hasn’t reached a final since lifting the Wimbledon trophy in 2013, looked to be in trouble as he 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 offered more drama than Murray would have wished for, and he was at a loss to explain his physical struggles.

“These Slams are physically challenging, but I need to work out why it happened,” Murray told an on court interviewer.

“It shouldn’t have happened, regardless of the temperature.”

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 holding for the match.