being 6 foot 6 helps. del potro has enormous easy power off the ground with that size.
he has also improved his serve so he is very difficult to deal with. poor guy is still without a single masters series shield.
that is just how tough it is today.
as for nadal, he is clearly spent. he has used up quite a bit of mental and physical capital in taking all that he has taken. winning 10 titles with such a demanding style of play would kill just about anybody else. he may have hit the wall.
this is why I cant see nadal taking anymore titles this year.