US Open 2013: Andy Murray faces Stanislas Wawrinka

Defending champion Andy Murray will face a player in the form of his life when he takes on Stanislas Wawrinka in the US Open quarter-finals on Thursday.

The ninth seed from Switzerland knocked out fifth seed Tomas Berdych in the fourth round and afterwards said: "I think I'm playing my best tennis ever."

Wawrinka, 28, was the last man to beat Murray in the first week of a Grand Slam - atthe 2010 US Open.Murray and Wawrinka are scheduled to start not before 18:30 BST.

But the British number one, 26, leads 8-5 in their head-to-head going into their latest meeting, this time on Arthur Ashe Stadium.

Wawrinka began working with Swedish coach Magnus Norman, a former world number two, in April and has returned to the world's top 10 in 2013 after five years away, picking up his fourth career title on clay in Portugal.

Arguably his most memorable moment of the year came in defeat as he took eventual champion Novak Djokovic to five incredible sets at the Australian Open in January.

"I don't know if he's working harder," said Murray. "He made a coaching change around the clay-court season, which may have helped him because he was without a coach for quite a while. That could have helped him a bit.

"The match he had with Novak in Australia at the beginning of the year, that could have contributed to it as well. There are a lot of things that go into it.

"But he hasn't changed any of his strokes technically or anything. You'd expect most of it to be confidence."

Wawrinka confirmed the theory, saying: "It is confidence, for sure. I had a tough beginning of the year, but in the same time it was really positive for me because I was playing great tennis.

Murray has yet to hit the heights consistently at Flushing Meadows this year, dropping two sets in his four matches, but the US Openand Wimbledon champion is a seasoned campaigner when it comes to peaking at the right time."I had some tough losses but I was still working out. I think the title in Portugal, beating David Ferrer in the final, helped me a lot with my confidence to pass the next level."

"When I was younger I used to play pretty consistent throughout the whole year, and when I hadn't won a Grand Slam these events became really my priority and I was always looking forward to the next one," the Scot toldBBC Radio 5 live.

"Sometimes I feel like it doesn't really matter how I do in the other tournaments because these are the ones I always get asked about.

"I try to peak at the Slams, I feel like mentally I'm a bit stronger at them than I was a few years ago, and I think the five-set distance suits me a little bit better than the best of three."

Murray's consistency at the majors was not lost on Wawrinka, who joked: "I think he's playing quite well in the Grand Slams too, since last year. Last year he won here and he won Wimbledon.

"He's really confident in five-set matches."

The remaining quarter-final will see Serbia's world number one Novak Djokovic take on 21st seed Mikhail Youzhny of Russia, not before 01:00 BST on Friday.