It will be interesting if they meet in Madrid because Rafa will have to up his level. Nishi's serve stats were insanely high in Barcelona (we're talking 92% vs 82% for Rafa) and his return starts were too (above 40%). He was also very aggressive, there is no way Nadal would hurt him in Madrid unless he raises his game several notches and that could actually be very beneficial for Nadal (to be forced to do that)
RG is a completely different story. There are major problems for Nishi at RG: stamina over a long match, higher bounce to handle. Not a good surface or event in general for Nishi.