- US Open
Nadal the man to beat - Murray, Djokovic
Nadal is currently 15-0 on hard courts in 2013, having won Masters titles at Indian Wells, Montreal and Cincinnati.
And Murray, who begins his first career major defence on Wednesday, admitted the Spaniard would be "very difficult to beat".
When asked if he was impressed with Nadal's form, Murray said: "It's hard not to be.
"He hasn't lost yet because he's beaten some tough players in tough tournaments, winning the two tournaments back to back in Cincinnati and Canada. I think it's only been done once before.
"I think Roddick did it once [in 2003], so it doesn't happen often. It's not like he's had easy draws or whatever. He's beaten some top, top players."
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World No. 1 Djokovic, champion at Flushing Meadows in 2011, said: "Rafa is definitely back and playing maybe the best tennis that he ever has played on hard courts.
"He's definitely so far the best player this year. There is no question about it. The results are showing everything."
"He won three Masters events and won them in great style. He was very aggressive, and he seems like he changed a little bit the game.
"He stepped in a little bit more. He knows that now he has to be a bit more aggressive than he usually is because of, I guess, his knees and everything and because hard court is not clay. It's not his favourite surface; it's faster.
"I'm sure he worked on that, and you could see that all the work he put in is getting results."
And Nadal himself agreed that his injuries have given him something to think about this season.
"I think you can play aggressive when you are playing well," he said. "That's the first and important thing. It's true I'm trying to take the ball a little bit early, that I worked on my game to be a little bit more aggressive."
Murray and Djokovic are both in the opposite half of the draw to Nadal at Flushing Meadows, so neither can meet the Spaniard until the final.