Novak Djokovic can't consider anybody but Rafael Nadal as his biggest rival in tennis due to the number of matches they played.
Djokovic has had rivalries with all members of the big four but for him, Rafael Nadal will always be the player he considers himself his true rival. It makes sense in a way because they are close in age and they have been battling each other for much of their careers.
Their careers look awfully similar right now as well with similar doubts and worries heading into this year's Roland Garros which is a crucial event in that whole story arc of the history of tennis. Djokovic is playing right now while Nadal is not but neither looks convincing according to some of the most knowledgeable names in tennis.
I can’t take anybody else but Nadal as my biggest rival. As long as Nadal is playing, as long as I am playing, Nadal is always my biggest rival. Regardless of the rankings or what’s going on, on the tour. Just because of the history of our rivalry.
There are many players that seriously challenged him over the course of his long career with even some new rivalries like that with Medvedev and Alcaraz emerging. But for Djokovic himself, Rafael Nadal will always be his biggest rival.
I have tons of respect for Carlos Alcaraz and some great matchups with Daniil Medvedev, but Nadal and myself, we go back a long time. So definitely, he comes in the front.
The next chapter in that rivalry might come at Roland Garros as they played against each other the last few times both attended. Fans would love to see it because it's becoming clearer and clearer that we're not going to get too many of their matchups in the future. Nadal's health is in doubt and even Djokovic is looking somewhat vulnerable recently.