By Alastair Himmer
"It's very difficult to always be 100 percent and what Federer did the last seven or eight years is almost impossible to repeat. It's unbelievable."
Nadal matched Federer in winning all four grand slam crowns by winning last month's U.S. Open to move to nine overall and add to the Wimbledon and French Open crowns he won earlier in the year.
"Of course he is going to challenge to win the big tournaments," said Nadal, who is making his first appearance in Japan this week. "He is still -- what? -- number two or three in the world and his game is still at a very high level."
Nadal and Federer are two of only seven men to have won all four majors and the Spaniard paid credit to the Swiss maestro.
(Editing by Ossian Shine and Pritha Sarkar)