The recent success of younger players on the men's tour points to an "inevitable" shift at the top of the ATP rankings, said world number one Novak Djokovic.
Russian Daniil Medvedev, 25, won the ATP Finals and the Paris Masters titles last year and has already displaced Spaniard Nadal from second spot.
Hubert Hurkacz, Stefanos Tsitsipas and Alexander Zverev have won the three Masters events this year with Greek Tsitsipas the youngest of them at 22.
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"There are guys like Tsitsipas, Zverev, (Matteo) Berrettini, (Andrey) Rublev that are winning against all of us and playing a lot and building their ranking points," he added.
"Medvedev as well, of course, and challenging for the top spots. Dominic Thiem has been there for many years.
"It's inevitable that it's going to happen. The change on the men's rankings, top of the rankings is coming. Whether it's going to happen in a month or a year or whatever, I don't know."
While the 39-year-old Federer missed most of the last 14 months due to knee problems, Djokovic passed his record for the most weeks as world number one in March.
But the Serbian, who has won 18 Grand Slam titles, is more focused on chasing down the 20 major wins of Federer and the 34-year-old Nadal.
"I'm not personally paying too much attention anymore to the rankings as much as I am to my game for the Grand Slams," he said. "Those are the biggest focus tournaments right now at this stage of my career."
Djokovic has made changes to his schedule to spend more time with his family and since his Melbourne win has only played the Monte Carlo Masters and an event at home in Belgrade.
He will wind up his preparations for the French Open, which starts at the end of the month, with this week's Rome Masters and another ATP event in Belgrade.
"Four tournaments before French is I think enough in terms of the match play," he said. "So I'm building my fitness and I'm building just my game slowly step by step in order to peak in Paris. That's definitely where I want to play my best."
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