Andy Murray admitted he had to be careful not to cave in to frustration during his victory over Leonardo Mayer in the second round of the US Open.
The defending champion was made to work hard for a 7-5 6-1 3-6 6-1 win over the Argentinian, who at 81 is ranked 78 places below Murray.
Mayer's forehand powerful proved to be a potent weapon and was a shot he used to dictate several points throughout the contest.
"I was a bit frustrated at points in the match because I was doing quite a lot of the running for a lot of it," Wimbledon champion Murray said.
"I wasn't getting much depth on my returns, so when he was serving I was having to do a lot of running.
"I served a low percentage (of first serves). I served pretty well in my first match, and then today struggled to control the first serve.
"So I was playing a lot of points on my second serve. When that happens, you have to do a lot of the running. You don't feel like you're dictating the match. It can be frustrating."
Murray was playing the second match of his campaign at Flushing Meadows on Louis Armstrong Stadium rather than the venue's main arena, Arthur Ashe.
He suffered his last early defeat at a grand slam on the court in 2010 against Stanislas Wawrinka, and struggled through matches with Robin Haase in 2011 and Feliciano Lopez and Marin Cilic last year.
Murray added: "It's a court I haven't played my best tennis on, that's for sure. And I think the conditions, well, it's day and night, isn't it? It's completely different.
"The ball was bouncing extremely high out there at the beginning, and he's a big hitter of the ball. He served big, too, bigger than I thought. Big second serve, too. It took me some time to get used to his game.
"I've had some tough matches there in the past, and today was the same."
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