querrey can try to win one on clay vs Tipsa, but Isner will lose both, and no one can beat Novak...easiest win here..
considering the way Djokovic and Querrey played recently, and the fact that Querrey lost 7-6 in third set to Gonzalez in Acapulco after just coming from his victory in Memphis whereas Djokovic will just arrive on clay with all the mental problems he showed last week,
I don't think it's impossible at all that Querrey gives many problems to Djokovic on first day.
Djokovic will beat Isner, but the Bryans are favorite, and the other matches are winnable for the USA. Serbia is favorite but I don't think this match is done yet.