The first half of Djokovic's 2011 season was the greatest ever, dominating the greatest claycourter ever (on clay) and arguably the GOAT as well. However he ran out of steam and the season doesn't end at the US Open.
Nadal's 2010 was great, but it wasn't dominance all year round. It was dominance in the final 3 biggest events of the year. He had too many losses and too few tournaments to be comparable to Federer's 2006.