2020 sounds about right. No one is really pushing them out. Rafa got his mojo back this year and is the likely candidate to end the year #1. Roger not looking bad either and still a threat at every tournament he goes in, could end #1 as well even though it's a bit of a long shot. Djokovic I think is slowly starting to find his form, same thing with Murray even though as usual, he's the weaker one among the 4 and had a mediocre first half. If you look at the rest of the top 10, there's no one there that has what it takes to overtake any of them, the consistency simply isn't there.