Comparing the years in which they both turned 34.
Slams:
AO: Federer lost in 3R to Seppi. Djokovic won it. Obviously, Djokovic was better. But if Fritz had held his head/was as solid as Seppi, he could've taken out Djokovic who was hampered in that match.
RG: Federer made it to QF, losing one set, but lost to peak Wawa in straights. Djokovic won it. Obviously, Djokovic was better. But 2015 RG Djokovic > 2021 RG Djokovic and given 15 Wawa beat 15 Djoko convincingly, he'd have beaten 21 Djoko convincingly as well.
Wim: Federer demolished Murray in the SF. Djoko had to rely on Shapo choking big time in the 1st 2 sets. Fed was also more dominant/impressive in the rounds prior.
Yeah, Djoko won his final vs a meh Berretini in 4 sets and fed lost to peak Djoko in 4 sets, but I don't think fed was any worse level wise in Wim 15 final compared to Djoko in Wim 21 final.
Their stats at respective Wimbledons are about the same overall despite Fed facing 15 Djok and 15 Murray, both of whom are significantly better than anyone Djoko faced in Wim 21.
Fed's serving stats a little better than Djoko's and Djoko's return stats a little better than Fed's
Serve: fed at 95.5% hold, Djoko at 93.8% hold. fed winning 75.7% service points, djoko winning 73.7% service points
Return: fed at 26.4% breaks, djoko at 28.3% breaks. fed winning 38.3% return points, djoko winning 41.3% return points
If we take stats before the final (Wim 15 final Djoko and Wim 21 final Berretini are like gigantic miles apart)
Serve: fed at 98.9% hold, Djoko at 94.6% hold. fed winning 78.6% service points, djoko winning 74.9% service points
Return: fed at 31.5% breaks, djoko at 28.6% breaks. fed winning 40.2% return points, djoko winning 41.7% return points
The service stats difference widens obviously. Fed also surges ahead on break%, even with a little less % of return points won.
Keep in mind that fed's competition before the final was significantly better: Murray, Groth, Querrey, Simon, RBA, Dzhumur
Djokovic: Shapo, Fucsovic, Kudla, Anderson,Draper, Garin
Conclusion? Fed in Wim 15 > Djoko in Wim 21, clearly
USO: Fed came into the final without losing a set, including dismantling Wawa in the semi. Lost to peak djokovic in a tight 4-setter in the final. Djokovic struggled on his way to the final and lost in straights to Med. Obviously fed was better.
Other tournaments till USO:
Federer in 15 won Cincy and Brisbane/Dubai/Istanbul/Halle, made the final of IW/Rome
Djokovic in 21 Won Belgrade, made final of Rome
So fed obviously better here.
Win loss record:
Fed was 51-8 till USO
Djokovic 44-6 till USO
Summary:
15 as a year was a
relatively weak one, similar to 06,10.
fed made 2 slam finals and a QF in 15 as opposed to djoko in 21 winning 3 slams and making a slam final.
so how much worse was 21 compared to 15? Quite a bit, I'd say.