Del Potro hits his forehand on the move so damn well.
Djokovic tries another ad court second serve into Delpo's backhand and Delpo takes it up the line for deuce. Djoker with a volley, an ace, a double fault, back to deuce, serve and volley forced an error, and an unreturnable serve.
If Djokovic wins, it's because of his serve. It's been spot on at the right moments. Both guys are playing the critical points well.