World number four Andy Murray has dismissed concerns over his fitness.
That had led some to question the Scot's ability to compete at the highest level for the remainder of the tournament at Flushing Meadows.
However, Murray has moved to allay any fears ahead of his round two match against Ivan Dodig.
"It was very, very humid on the court today," he told reporters. "Maybe I didn't take enough fluid. It happens.
"I haven't played that many matches in that sort of humidity for a while. It's probably a bit of a shock to the body. I need to make sure I'm on that for the next match."
Murray's previous best in New York saw him reach the final in 2008.