World number 10 Stanislas Wawrinka has cited his growing confidence as the reason behind his recent good form at the US Open.

The Swiss meets defending champion Andy Murray in the quarter-finals on Thursday in, arguably, the form of his life, having beaten Ivo Karlovic, Marcos Baghdatis and Tomas Berdych already at Flushing Meadows.

Wawrinka, who took world number one Novak Djokovic to five sets at the Australian Open earlier this year, thinks that his tough start to the calendar prepared him well for the remainder of it.

"It is confidence, for sure," Wawrinka said in a press conference. "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.

"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."

Murray leads 8-5 in head-to-head meetings with Wawrinka.