The two longtime rivals witnessed career resurgences in 2017 as they won a combined 13 titles and split the four Grand Slams between themselves.
Nadal notably ended the year as the world number one ahead of Federer, but he suffered four losses to the Swiss legend, three of which came in finals, including the 2017 Australian Open.
The Spaniard, who boasts 16 Grand Slam titles, will have to end his recent losing run against Federer should they face in another final if he wants to catch his tally of 19 Grand Slam titles.
With Federer currently playing at 36 years of age and already hinting at retirement in recent months, Uncle Toni told Spanish outlet ABC: "I trust that Rafa will last long enough to beat Federer's record. Next year it will be difficult to win many titles.
"Although I hope Rafael continues to prove he is the best on clay and win again at Monte Carlo, Barcelona and Roland Garros."
It is not the first time Toni has spoken about his nephew catching Federer. In October, he said Nadal could even be considered as the GOAT of tennis over Federer if he wins more Grand Slams.
"Rafa is playing at an exceptional level, just look at this year's results, and I am totally sure he will be among best players ever," Toni stated in October.
"If Federer retires within a few years and Rafa keeps winning Grand Slams, then we will speak about GOAT question but at the moment I think it's Roger."