Roger Federer is the biggest of Grand Slams' 20-year-olds.
However, hot pursuit is Nadal (17) and World No 1 Djokovic (14).
Fighting in the top 2018 world rankings saw that all three players occupy no. 1.
But Djokovic will be the one who finished the year post after Nadal's withdrawal from the ATP Finals.
Federer lost his opening match against Kei Nishikori on Sunday night, but his coach, Severin Luthi, believes the Swiss veteran still has what it takes to send Djokovic.
And he thinks Serbs are harder to beat than Nadal at this time.
"It changes again and again. At first I thought Djokovic could not beat Nadal," he told the Swiss newspaper Tages-Anzeiger.
"Then he came back, because I thought, conversely, that it was not possible.
"Sure, Djokovic is again the number 1, he has played in the excellent summer. It is now hard for anyone to beat.
"He is very solid, complete, very strong in the defensive.
"But he has nothing unusual that makes Roger great. If Roger plays very well, he can beat him.
Meanwhile, Sky Sports researcher Annabel Croft, who saw Federer fall for a direct defeat by Kei Nishikori in their London opening match, has expressed concern over the 37-year-old.
Federer – who was outside of him – hit 34 unforced errors and received a warning from the referee to break a ball in the stands.
"Under the surface are nerves and nerves," Croft said.
"When he goes there, there's a little chips coming in and we're not accustomed to seeing him. He's not running freely.
"Stuttering and thinking more, even on basic core points."
Federer is the next in action against Dominic Thiem tomorrow night.