Nadal, who with Federer has swept all but two of the past 23 Grand Slam titles, said it was becoming increasingly difficult for the top two to maintain their stranglehold as new players come through.
"In more than 20 Grand Slams, only two or three players won Grand Slams. I think that's impossible to continue like this," Nadal told reporters before the Australian Open starts on Monday.
"I think that's not going to continue like this. We will see what happens but there are a lot of good players, a lot of young players, and every year is more and more difficult."
The Spaniard also admitted his quest to unite all four Grand Slam titles -- for the first time since Rod Laver in 1969 -- was "almost impossible".
"I think it's almost impossible. It's very, very difficult," he said.
"Tennis is a very competitive sport and there's not a lot of difference between players. So a lot of matches are decided in a few balls. So for that reason is very difficult to have one player winning everything."
The 24-year-old already holds the French Open, Wimbledon and US Open trophies after recovering from an injury which forced him out of last year's Australian tournament.
Nadal added that he was recovering from an illness which struck him down at the Qatar Open, but was yet to hit top form. He will play 96th-ranked Marcos Daniel of Brazil in the first round.
"I'm feeling better. I think not perfect yet. But, yeah, it seems like after what I had in Doha, when I'm practising, I feel a little bit more tired than usual and sweating more than ever," he said.
"But the truth is I'm better than a few days ago. So that's very positive. I hope it's not going to be a problem for Monday or Tuesday."
Nadal and Federer, 29, have been named as joint favourites for the Australian Open, where the Swiss collected his fourth title last year. Novak Djokovic and Juan Martin del Potro are the only other major winners since 2006.