Obviously this is based on Nadal staying healthy until the USO, we all know he tends to disappear when the indoor season kicks in so he needs to perform this summer. I can see him lifting another trophy in Flushing Meadows if his body holds. Normally I would go with Djokovic as #1 and Nadal as #2 but I think Djokovic has shown that he's a bit more vulnerable this year on hard courts. I'm still puzzled by his subpar performance in the 2 tournaments I consider his strongholds (IW & Miami), so I'm not too sure how he will do in Montreal & Cincinnati, the USO has always been shaky even in his better years due to the weather/humidity. I expect Federer to make a deep run at Wimbledon and maybe win, maybe another Cincinnati title if stars align, but the USO hasn't been kind to Federer in recent years. I think Thiem has shown a bit more consistency this year and that's why I think he'll finish ahead of Tsitsipas, heck I could see Thiem overtaking Federer at #3 at some point too if he makes a deep run at the USO.
Djoker has A LOT to defend from now till the end of the year, while Nadal only played 3 tourneys last year from July-November, which is why I give him a chance at snagging the number one ranking if he can continue winning, stay healthy, and Nole falters at some of the tourneys he won last year. I feel like Zverev can turn it around, although he's looked pretty terrible this year so far. Stefanos is ahead of schedule right now and I'm not sure if he can sustain this play for another half of the season
Depends on who wins Wimbledon. If Nadal & Djoko go very deep (final 4 or Final) then it's between them for Top 2 spots most likely.
But if Fred wins the tournament he could really make things interesting at the top and sneak in as well