If you use the Grand Slam Stakes as the metric by which to measure tennis players’ skill, then Novak Djokovic is the best. As of April 2020, he is a mere one win behind the master Rafael Nadal, and is a favorite to make that win and one more, thereby taking the Masters Series title from him. This would make him the best tennis player of all time officially. He already had a season with 95% of his matches being wins. This is a higher percentage than his main competition, Rafael Nadal and Roger Federer.  He is predicted to spend more weeks at No. 1 than competitor Roger Federer in 2020, which would give him another concrete statistic attesting to his greatness. Just because Djokovic has not yet beat all of the records held by other greats does not mean he isn’t already the best. He has the skill, talent, and potential to beat them all already. This means he is already the best tennis player of all time, but merely still early in his career. Another reason for his still comparatively low record of wins is that he has spent much of his career playing against other tennis greats. This means the level of competition faces is high. This steep competition means Djokovic can (and is) playing with more talent and finesse than anyone else, but has merely not had enough time to bring home the most prizes of anyone. By measuring tennis strength with an internal measure (i.e. watching the quality of play) instead of an external one (i.e. number of wins), you gain a more accurate and long-lasting concept of who the actual best tennis player is. After doing so, Djokovic clearly stands as the best tennis player of all time.
Predictions about Djokovic’s future success do not make him the best tennis player. Only time will tell if he will be. Saying he’s the best tennis player of all time now is premature and thereby incorrect. The best tennis player should be determined by his/her number of wins. That’s how it is all throughout the sporting world; Djokovic is no exception. The goal of a sports game is to win. If your talent and skill don’t translate into wins, you’re not the best, because you’re not achieving the objective. Finally, Djokovic is arguably (possibly) the best male tennis player. Sports often choose male and female greats, but there can only be one all-time-best-tennis player, and there is formidable female competition such as the Williams’ sisters.
[P1] Tennis prowess is measured in titles & medals. [P2] Djokovic is on track to have the most titles & medals ever. [P3] Therefore, Djokovic is already the best tennis player ever.
Rejecting the premises
[Rejecting P2] Djokovic could fail to achieve the most titles & medals.