US Open finalist Roger Federer is playing at close to his best ever level, according to Stan Wawrinka.

Wawrinka proved no match for his Swiss countryman in their semi-final on Friday, losing 6-4 6-3 6-1 at Flushing Meadows.

Comparing 34-year-old Federer's form to previous years is a difficult task, with his first grand slam title having come in 2003.

But Wawrinka said the 17-time slam winner was approaching his highest level.

"It's tough to say. He's been winning so many grand slams, so many tournaments. We'll see. For sure it's close from his best," he said.

"Also the game now is a different level than few years ago, so everybody is improving from few years ago. But it's the best that I saw him play since [a] few years, that's for sure."

Wawrinka was expected to test Federer, who will face Novak Djokovic in the final, in their last-four clash.

But the French Open champion never seriously threatened after Federer made a strong start and Wawrinka paid tribute to his countryman.

Asked what had been so difficult, Wawrinka said: "Him, the way he's playing.

"Condition a little bit different than when I play my [last] match here on Ashe? It's a little bit more flying in the night. It's playing really fast. Didn't play my best game. Didn't serve well and everything.

"But basically it's him, the way he's playing."