US Open: Andy Murray in good shape to face Jo-Wilfried Tsonga

Andy Murray believes his physical problems from early in the tournament are behind him as he heads into the second week of the US Open.

The Briton beat Andrey Kuznetsov 6-1 7-5 4-6 6-2 on Saturday to set up a fourth-round clash with Jo-Wilfried Tsonga.

And he is optimistic the cramp which afflicted him in his opener against Robin Haase is no longer an issue.

"I feel like I'm in very good shape," he said.

He added: "I trained extremely hard in Miami to get ready for this period of the year. It shouldn't be a problem."

The conditions were considerably kinder for Murray's return to the Louis Armstrong Stadium, five days after his gruelling encounter with Haase.

"Today was much better than Monday," he said. "It was obviously cooler and may have been a little bit more humid.

Andy Murray has reached the US Open fourth round for the seventh time in 10 years

"It was quite cool actually by the end of the match. I hope [the cramp] is not a problem for the rest of the tournament."

Murray has not always enjoyed playing on Armstrong, the second show court at Flushing Meadows, and at one point nearly collided with a TV camera.

"I almost ran with my knee straight into the camera at the back of the court," he said.

"If I was a spectator, and a player had to stop because he got hit by a camera that was too close to the back of the court, then I would be annoyed if I was watching that."

He added: "It's a very tight court as well. There's not much run-back compared with Ashe (Stadium). Outside the lines there's not that much space."

While Murray dominated for much of the match, world number 96 Kuznetsov did break serve four times and managed to force a fourth set.

"I expected him to play well," said Murray. "He obviously would have been confident from his last match.

"The first couple sets were pretty good, and then I started a bit slow in the beginning of the third.

"He gained confidence from that and he definitely started hitting the ball better in the third set. It was a tricky match but I came through well."

Murray is likely to face a far sterner test against ninth seed Tsonga, with the Frenchman having won their last encounter in Toronto earlier this month ago before going on to win the tournament.

"We've played each other a lot of times," said Murray.

"He's obviously playing well right now. He had a great week in Canada. He's played some good tennis so far here. It will be a tough match."

Andy Murray has reached the second week of a grand slam for a 15th straight time