Ferrer crushed DelPo at Wimby.
At the Olympics DelPo lost to Federer - who was thrashed by Murray. Djokovic was not thrashed by Murray. Djokovic's level is much higher on grass.
that's not really a good way to look at it though
ferrer frustrates del potro, gets spanked by federer, who loses to nadal, who gets spanked by djokovic
is djokovic better than all those on all surfaces? no
on hard court, djokovic is too good. on grass, del potro is the tiniest bit better. on clay...that's the match i want to see