That is the opinion of Nadals former coach and uncle Toni, who believes the likes of Alexander Zverev and Karen Khachanov can make a mark in the Grand Slams next year.
Federer, Djokovic and Nadal have dominated the mens game over the past decade, winning a combined 51 Grand Slams.
But a new generation of players have begun to show their potential, with Khachanov beating world number one Djokovic on his way to winning the Paris Masters earlier this year.
Zverev, 21, also caused an upset in the season-ending ATP Finals, beating Federer and Djokovic in straight sets to clinch the biggest title of his career at the O2 in London.
Toni Nadal told Diario Vasco: I said from the beginning that Djokovic was going to be on form again [this year] since his quality was so high.
I was surprised that people stopped thinking of him so quickly. In 2019 it may be the generation change that is being pointed out is finished.
The young players have taken an important step this year. Karen Khachanov was able to prevail over Djokovic in the final of the Paris Masters.
Alexander Zverev has also imposed himself on Nole [Djokovic] in London. [Denis] Shapovalov is the youngest but can move forward as well.
There is a very strong group with [Borna] Coric or [Dominic] Thiem who are about to make the final leap.
Rafael, Djokovic, Federer know that they will have to play and beat these young players.
Toni Nadal also believes two experienced players in Juan Martin del Potro and Andy Murray could trouble Federer, Djokovic and Nadal next year.
Both players will hope to find their top form in 2019 after recovering from injury problems.
Juan is another player who has had constant problems and sometimes we forget him because he has lacked continuity, he added.
It is complicated to play with so many problems. But like [Andy] Murray, if they recover, they will be very difficult rivals to beat.