He has a great chance but on a given day you never know!
The fact of the matter is that players with a big game can take it to the big players! Isner showed this when he took Djokovic to 5 sets in the Davis Cup on Clay!
He backed this up last year by winning against Djokovic in IW with a display of huge hitting and huge serving! Which even Djokovic one of the best returners in the world could not handle!