All Nadal can do is hold serve and hope for a small chance otherwise it`s the tiebreak lottery
tiebreak is not only a lottery : differences in percentages between players are quite obvious.
Isner is one of the strongest for that : as Voo de Mar said, he's won nearly two thirds of the one he played, only Federer has done little bit better (Karlovic is not at all, which proves that it's not only about serve, it's about mental strength). Djokovic is also very good for that.
I consider that Nadal and Isner may be the strongest mentally players on tour, then it can be tough (but in tiebreaks Nadal doesn't have such great stats, little bit more than 50% only)