World number two Andy Murray conceded he "did not play well" but was still happy he advanced to the last 16 of the US Open.
Murray was troubled early but was able to see of Paolo Lorenzi 7-6 (7-4) 5-7 6-2 6-3 in New York on Saturday.
The Italian put up a fight, but fatigue ultimately cost him in his challenge to knock off the Olympic Games champion, who will face Grigor Dimitrov in the next round.
On Lorenzi, Murray said: "He's a very good player. He does a lot of things well on the court.
"He has good feel, moves well... It will be another tough one [against Grigor Dimitrov] in a couple of days' time. I expect it to be very hard.
"I'll be ready for that. I'll need to play better than today if I want to win that."
Murray continued: "I expected a tough match. I expected long rallies. I'm just disappointed with the amount of errors I made.
"I was quite impatient at times. That cost me in the first and second sets.
"I worked it out, won, got through, and I didn't play well. So I'm happy with that."