The only thing Nadal improve significantly, is his serve. It take him 2 semi-finals in US Open to realize he must have big serve to win here. So he trained for this since last year -- because to win US Open is his first priority and most important, just like he said.
His serve has been getting better since early this year (see my post in May), and the speed has been improving from 110mph, to 120mph in May and June, and to 130mph in August. it is not 2 months.
he used to be clay-expert, and people say he can't win on hard court and grass court. then he trained hard, and after 2 years final-lost to Federer, finally won the grass grand slam -- it is not 2 months, it took him 3 years of improvement to win on grass and beat Federer.
nadal's success, is hard-work + telents = mental toughness, not doping.