Fine I'll give my thoughts on nit picking Fed's AO '06. Davydenko played at a similar level to Zverev, except Davydenko engaged in less choking. I think those matches both went about equally so will call it even. Davy better from the ground, Zverev when good was botting at a high level but also coughing up awful DFs/UEs so it evens out.
Kiefer v. Karatsev: well Kiefer was actually a really nice all-court player with a strong serve, forehand, and willingness to come forward. He also took sets from '05 Fed at Wimby/USO. Not a mug by any means. Remember that Medvedev dropped a set to qualifier Botic Van de Zandschulp at the USO. Dropping sets to guys who can play attacking tennis with precision is not really that big of a deal.
Still, Kiefer match was very much NID. Baghdatis match on the other hand was anything but NID, but then his legs gave out. I'd classify that one as where I think Fed was a little flat in the final and was maybe a little fortunate to not go down 0-2, but his 3rd set (while Baggy was still trying) was probably the best he played all tournament. This is still '06 Federer we're talking about guys, his FH was demonic and Fed started to time the Baghdatis serve, it was destruction.
Overall I think AO '21 Djokovic
could beat Fed if they met in the final but the fact is Fed could outlast this Djokovic from the baseline. It's just hard for us to picture it this way due to the images of Fed being older than Novak, but in a situation where the ages are flipped, Djokovic would be forced to end points earlier and may struggle. I do think Djokovic would pull it out if he got the first 2 sets, but if it was 1-1 going to the 3rd I'm betting Federer wins it. Fed's level raise in set 3 of the final is being underrated.
This is a WTA account now.