Of course he probably benefited from more training on these courts than other players.
maybe he was favoured playing more on center court last week (Fed said he hand't played on center court yet last saturday, I think), but he had said that he had refused to practice here before.
Probably Nadal knows better how to slide, which shows that good clay abilities are still useful
Also grass abilities maybe as Nadal also moves better on slippery grass than Djokovic
Anyway, call it the rich's luck if you want, but in round 16, apart from Isner, the tournament has all of the players who were supposed and being able to reach R16 actually.
And I've seen very good tennis so far.
Madrid had already been lucky in the past to have Nadal-Fed meetings every year