Roger Federer, 37, claimed his 20th Grand Slam earlier this year at the Australian Open.
That came on the back of winning two in 2017 after going six years without one.
The narrative of the last two years in tennis has been based around comebacks with Federer, Rafael Nadal and Novak Djokovic all returning from injuries to get back to their best.
Djokovic has finished the year as world No 1 after winning two slams – and of the past eight Federer has three, Nadal has three and Djokovic has two.
Stan Wawrinka, Andy Murray, Marin Cilic and Juan Martin del Potro are the only players to win slams other than the main three since 2005.
But Rafa’s uncle Toni thinks Federer could be finished in the top events, because of his struggles with playing best of five sets.
Federer is the leading male Grand Slam winner of all time with Nadal four behind and Djokovic six.
But Toni Nadal thinks the likes of Alexander Zverev will begin to challenge in the top events – after the German beat Djokovic in the ATP Finals, after beating Federer in the semis.
“What can we expect from 2019? I find it hard to see Federer raise another Grand Slam cup,” he wrote in a column for El Pais.
“Obviously, I do not say it for their game, although I do say it for the hardness of the five-set tournaments. I have to confess that I said that on other occasions and the Swiss, repeatedly, surprised me.
“Players of the Next Gen has continued to grow and improve. I think we have seen, at last, a quality leap in his (Zverev’s) game.
“I understand that there are many of us who see the definitive consolidation of players like Zverev or Thiem, although for obvious reasons I want it to happen later than soon. I would gladly wait for them to be delayed a little longer.
“On a regular basis, new generations, in all sports, have surpassed the records of their predecessors. Michael Phelps beat the times of Ian Thorpe; Usain Bolt, those of Carl Lewis and those of Ben Johnson.
“For some time I had my doubts about whether the youngsters would be able to overcome the game of Federer in grass, the one of Djokovic in fast track and the one of Rafael in beaten earth.
“And I had the impression that the relay would be caused rather by the decline of some than by the attack of others. And do not want to be understood in these words lack of admiration and recognition because there is not.
“The final in London has made me think about it. For now, for the coming year, I trust in the full physical recovery of Rafael, I feel new episodes of his rivalry with Djokovic and, in general, a more open panorama with the definitive incorporation of the new generation.”