Continued:
2015 vs. 2020 vs. 2021:
AO: Djokovic
RG: Nadal
W: Federer
USO: Federer
YEC: Federer
Explanation: Looks like a clean victory for Fed here but this is one instance where this representation is a little misleading. I would certainly take 2015 Fed over 2021 Djokovic at Wimbledon and the US Open, but those margins are narrower compared to Djokovic's lead over Federer in the first two Slams, although Nadal still trumps Novak at RG.
Wimbledon might be a controversial pick but you have to realize that 2015 Djokovic is basically Djokovic's top level at Wimbledon. Serve was great, baseline game was super sharp, and this was one of his personal best returning performances imo. Still, Fed ran it basically dead even for two sets and a half on the back of some good serving (though not as good as 2014) and some very clutch play at times. I think he should definitely be favored in level over 2021 Djokovic who played a bad first set and needed more time to get into his groove... and not to mention he also returned worse than in 2015, which will hurt him. The only argument I could see for him here is one based on stamina. Fed did start to run out of gas in the tail end of the third set while 2021 Djokovic had no such issue, but, again, Fed was keeping it level with peak grass Djokovic for a good long time so it's understandable.
I'd also give the YEC to Fed. Certainly a forgettable performance in the final but I don't think Fed would have lost to someone on the caliber of 2021 Zverev.
Nadal at this point is starting to fade away from the other two.
2016 vs. 2021 vs. 2022:
AO: Federer (*)
RG: Nadal
W: Djokovic
USO: ---
YEC: Djokovic
Explanation: This year is pretty ugly because I wouldn't exactly classify any of these guys as being particularly in form, plus all three skipped the US Open at the same point in their careers so we have no data there.
The whole deportgate thing obviously looms over the AO result but in the absence of data from Djokovic, I'd have to go with Federer. Fed didn't play a very good match in the 2016 semifinal, but Djokovic basically hit god mode there so he was made to look a lot worse than he actually was. We should be praising him for taking a set honestly. I would personally take this over losing to Tsitsipas in the QF.
RG will be a closer contest because 2021 Nadal and 2022 Djokovic weren't really in form. I do rate the 2021 match higher though, so that has a hand in my choice of Nadal here.
Wimbledon is definitely Djokovic. Fed led an inspired charge to the semifinals... but he was injured. Djokovic was in iffy shape going into the final but I thought the final itself was actually a good performance; there's not as many holes to poke here as you would with 2019 or 2021, plus the returning was legitimately impressive by a general standard, not just for someone in his mid-30's.
Djokovic also gets the YEC since the other two are AWOL.
2017 vs. 2022 vs. 2023:
AO: Federer
RG: Nadal / Djokovic
W: Federer
USO: Djokovic
YEC: Djokovic
Explanation: Like with Djokovic's 2021 season, a little deceptive because on aggregate, I think Djokovic had the best season, but this doesn't materialize in the Slams because Fed has enough to edge him at the AO and Wimbledon (while doing little at RG and W) while Nadal can stop him at RG (while doing little damage at AO, W, or USO).
There are a couple of controversial picks here. The first is the AO. Personally, I think 2017 Fed just hit higher highs in overall level (particularly in the fifth set of the final), took down much stronger opponents, and wasn't carrying an injury. For these reasons, I pick him here over 2023 Djokovic. And sorry guys but I just do not rate AO 2022 Nadal at all. I think he could do some damage to Fed or Djokovic but honestly that was one of the lowest level Slam victories I think I've ever seen.
The second one is RG. I think we can agree that Nadal's level at RG 2022 was the lowest of all the RGs he's won. Personally, I would back 2021 Djokovic in a potential encounter. However, I also don't rate 2023 Djokovic as high as 2021 Djokovic. He had a few struggles along the way and barring Alcaraz's cramping I believe that would have been a difficult (though winnable) match for him, in the same way that Nadal would have been troubled even more if Zverev hadn't had that freak injury. However, I think Nadal would have performed better in more favorable conditions because they were pretty terrible in that semifinal. When it came down to it, both of them curbstomped Ruud in the final. I have a preference for Nadal's overall performance in the tournament and I do believe he had the tougher field, so I went with him, but you could very easily go with Djokovic here which is why I've also included him. I think the other picks are pretty open-and-shut.
So this is where we finally run out of data for Djokovic and will have to wait before we compare his 2024 season with Fed's 2018 season and with Nadal's 2023 season.
Full Summary:
Fed builds up a very, very strong lead because of his great 2011 and 2012 seasons. These two blow every other season from all three players out of the water (the only one that would even come close in my estimate would be 2018 Djokovic, but that was for half a season). Certainly, he's the best of the three in their early 30's, and he continues to be very much relevant in their mid-30's despite a slump in 2013 and part of 2014.
Djokovic has a horror start to his 30's but later sees improvement relative to Nadal and Federer, and I'd say he edges them in their mid-30's. Still, there are a couple of fluctuations - 2019, 2020, and 2022 weren't quite as strong as 2018, 2021, and 2023 from a Slam perspective - that hold him back. Another thing that holds him back are interesting circumstances like the Wimbledon cancellation of 2020, throatgate, deportgate, and the US Open COVID ban in 2022. As I mentioned above, I don't think Djokovic's participation in Wimbledon 2020 would change things, but for the other three Slams mentioned, it probably could. Plus, as I've hinted at times in this post... Djokovic often made a strong second place even in Slams where he was not quite the #1 of the three. 2018 and 2019 Wimbledon, as well as 2021 RG and 2023 AO, are certainly among those. As such, I think his final tally definitely underestimates his longevity.
Nadal, like Fed, does very well in his early 30's (2017-2019 carries hard) but fades away in his mid-30's, although his pure superiority at RG is enough to keep him afloat and continue to add to his tally even then.
In total, here is the list of how many individual Slams I believe each of the three was the best at during their 30's:
Federer: 12
Djokovic: 6
Nadal: 9
Here's the YEC:
Federer: 5
Djokovic: 2
Nadal: 0 (some things remain constant...)
Again, that's my subjective take. There's definitely wiggle room, especially regarding a few of the picks I made in the lists above.