We saw them peak for peak in 2011

2011 Nadal reached more finals than 2010 Nadal. He didn’t win as much because he faced Djokovic
Nah, wrong on both counts.
Djokovic was good in the final but worse than in the semis; as I said, it was the bad serving. Nadal played great for one set, but the others were far too straightforward and his shots weren't nearly as explosive.
Especially the serve. Ned's serve was a brutal weapon in the 2010 tournament while 2011 final is a strong contender for the worst he's ever served in a Slam final.
WRT the second point... what if...
both Nadal declined and Djokovic improved? The two don't have to be mutually exclusive, you know.
And I think that's exactly what happened. Especially on clay, because 2010 Nadal swept the whole clay season in remarkable form while 2011 was hit or miss by his lofty standards. Event-by-event comparison:
AO: Draw, both were injured I think
IW: 2011
Miami: 2011
Monte-Carlo: 2010
Madrid: 2010
Rome: 2010 (this tournament was devastating by Nadal; I think he dropped something like only 25 games along the way)
RG: 2010 (hit-or-miss in 2011, several of the matches there were quite low-quality by his standards, while 2010 didn't drop a set at all)
Wimby: 2010 (Nadal was bang-average in 2011; a very middling final from him and a bit worse than the 2006 final imo)
Canada: 2010 (Nadal lost super early in 2011)
Cincinnati: Both were poor but 2010 I suppose
US Open: 2010
Shanghai: Again, both were poor but 2010
Paris: Nadal skipped both tournaments
WTF: 2010
Overall it's not even that close. 2011 Nadal reached a lot of finals but he never delivered in the same way that he did in 2010.