Djokovic was only 20 yo when he played the final of the USO. He was still 20 yo when he won the AO back in 2008. Djokovic had reached the semis at W before retiring against Nadal. Djokovic was also getting to semis of RG etc.
I'm not going to talk about masters and the fact that he was able to def. Roddick, Rafa and Fed in Montreal. #3, #2 and #1. That was in 2007. That is, for younger guys, when people start calling them 'the big 3'.
Zverev is not even close. I know some of you are 'in love' with Zverev but this is just silly. Zverer achieved one QF at RG and he keeps losing in R1, R3, R4, R3, whatever.