Andy Murray is still unsure why he is cramping in matches after results of sweat tests at the US Open did not indicate a serious issue.

"The sweat test was good, blood test was good. No illnesses," he said.

"I know now it is down to conditioning, hydration or food related."

Speaking at the US Open on Saturday, the world number 49 added: "I feel like I am in decent shape, so that is why it is concerning. The results of tests were positive news but I don't really know why it is still happening."

Murray, 35, returns to the US Open - on the 10th anniversary of his maiden Grand Slam victory there in 2012 - when he plays Argentine 24th seed Francisco Cerundolo on Monday.

The Scot says he will monitor his food and liquid intake close to matches, saying he might have to be a "little more precise with that stuff" in hot temperatures in New York.

"Usually I am pretty precise and good with it, but it is a fine balance to make sure you're getting the right fluids when you're on court," said three-time major champion Murray.

"I take litre-and-a-half bottles on to the court to make sure I'm drinking enough at the appropriate times. In these conditions I always try to drink a litre and a half of my sports drink in 40 minutes.

"But if you have two games which last for 15 or 20 minutes then it is difficult to make sure you're drinking enough. If you take on too much you can feel bloated.

"The last few days in practice I have actually felt a bit better in the conditions, so hopefully I will be all right on Monday."