Andy Murray has admitted that winning his US Open fourth-round match against Denis Istomin was "tough".

The defending champion got the better of his Uzbek opponent 6-7(5) 6-1 6-4 6-4 to progress to the quarter-finals of the tournament.

"It was definitely tough," BBC Sport quotes him as saying. "It was pretty breezy out there, the wind was quite strong and both of us struggled without timing early on. When it's breezy like that, you can have some entertaining points.

"After losing the first set, I started trying to dictate the points, especially when the wind was with me, and use my forehand more. I had the chance to win the first set so it was just about minor adjustments."

Murray will face Stanislas Wawrinka for a place in the semi-finals.