One among Roger Federer, Rafael Nadal and Novak Djokovic will win the Grand Slam Race and, at the end of his career, will become the most successful tennis player ever. This is a statement that has no doubts; no one can reach them now, hardly anyone will succeed in replicating their titanic achievements in the future.
Roger Federer is ahead of his rivals. He has won 20 Slams, which is a record today. Age is the most disadvantageous factor. Or maybe not? At 38 years-old Roger still plays wonderful tennis, which none of his younger rivals can match.
Physically he is still intact. Rumors talk about his eventual retirement in 2020, but perhaps Federer will be able to surprise us again and get to play until 2021. Of course, every opportunity left behind is lost, like the enormous chance he had at Wimbledon.
He will probably still have a remote chance of winning the Australian Open or Wimbledon 2020. Difficult, but not impossible. In the most optimistic forecast, he could close his career at 21 or 22 Majors. Nadal is younger than his rival, and at 33 years-old he may still have another 3 or 4 years of high-level tennis ahead of him if his physique will withstand the shock.
He is a step away from Federer with 19 titles. Rafa could win the next two editions of the French Open with just one hand. He will also have chances at the US Open in 2020 and in 2021. Theoretically, Rafa could close his career with or 22 or 23 Slam titles.
Novak Djokovic is the youngest of the Big Three (32 years). He has a total of 16 Slams. Four are missing to get to Federer's 20, but he will have his chances to match his two rivals. At the Australian Open 2020, at Wimbledon and US Open 2020 he will certainly be among the favorite (except for injuries).
Moreover, considering also his physique, Nole could have at least another 3-4 years of high-level career ahead of him. He can close his career with 20 or 21 titles. In conclusion, I can say that the margin is really thin; many variables will have to fit together so that one of the three prevails over the other.
Anyway, considering titles, age and perspectives, Rafael Nadal could be the one who can close his Slam-career ahead of Federer and Djokovic, and then win the Slam-race.