After the game is before the game.

(Nach dem Spiel ist vor dem Spiel)

Always loved the above saying by Josef "Sepp" Herberger (quoted at the beginning of Run Lola, Run).

Looking it up I came across this one which is also pretty great:

The next game/opponent is always the toughest one.

(Das nächste Spiel/Der nächste Gegner ist immer das/der schwerste)