Don't write off Roger Federer yet, warns Rafael Nadal

World number one Rafa Nadal insists his great rival Roger Federer is no spent force and on Wednesday backed him to add to his record haul of 16 grand slam titles.  

"He won at the Australian Open and won in Cincinnati (Federer's 17th Masters 1000 title), got to the final in Madrid and Toronto ... so it's a good year for him," Nadal told reporters at this week''s Japan Open.  

"It''s very difficult to always be 100% 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 US Open to move to nine overall and add to the Wimbledon and French Open crowns he won earlier in the year.        

But the Spaniard warned that it was far too early to write off Federer, who has won 63 career titles but has slipped to third in the world rankings.     

"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.      

"It's very difficult to always be at the top," said Nadal, who is gunning for his seventh title of the year this week. "What he did is very, very difficult."