US Open 2014: Andy Murray says reaching second round a bonus

Andy Murray says he is "playing with the house's money" after overcoming cramp to beat Robin Haase in the first round of the US Open on Monday.

He returned to the practice court 24 hours later, and will play qualifier Matthias Bachinger on Thursday.

"I was lucky to come through [on Monday], I could easily be on the plane home," the Briton told BBC Sport.

"So I'm playing with the house's money and I'll just try to go out there and do the best I can."

Murray, seeded eighth in New York, had been at a loss to explain his bout of cramp after the match and remains unsure of the specific issue.

"It can be a combination of a number of things or it might be one thing in particular, so I just need to do all the right things," he said.

"I don't play my next match until Thursday so I'll have time to hydrate and get enough fluid in the body, and eat as best I can."

Murray lost nine games in a row at one point against Haase and looked in real danger of his earliest Grand Slam defeat since 2008, before coming through in four sets after three hours and eight minutes.

The temperature pushed 30C on Monday and is expected to be high again on Thursday, but Murray does not believe the cramp was down to his physical conditioning.

"I felt great before the first match and that obviously happened, so you never know what's going to happen each day," he said.

"It's been hot and tricky conditions so far. I felt physically ready, it was just a strange situation.

Murray struggled physically from the start of the third set against Robin Haase

"Hopefully I'll pull up fine over the next couple of days. Before I went to bed [on Monday] I felt fine, I woke up with no aches or pains or stiffness.

"That suggests it was more down to something I hadn't eaten or drunk, or I was lacking in something before I went out there, rather than me physically not being in good condition."

Murray will play Germany's Bachinger, a qualifier ranked 235 and former rival from junior days, for a place in the last 32 on Thursday.