Agreed, needs to get battle hardened. I thought he'd beat Rosol though... I guess he's not quite there yet. Maybe he was also feeling the Berdych match in his legs too. Tomas retired today.
I do not think physical state of body was case of his loss. Rosol was in the same situation, he played 5 sets on Friday too. Only difference was that Rosol is more experienced. He played more matches in Davis Cup and of course the difference between must and can is huge. Zverev was considered to be favourite and Rosol was underdog in this case, however this decisive matches in Davis Cup (the atmosphere on courts with all the fans rooting for thier country) is so different to "common" ATP tournament.
I am just guessing, that on ATP tournaments, even in Davis Cup if it was not last match and if it was not played in Germany. Zverev would won in 4 of 5 apperances against Rosol. But it is just my guess.
Now Zverev knows that the road ahead to the top is very very long and Rosol knows that he can win against better players. Will see how they yield this match in future games on ATP tour.