Well, actually G2 didnt play well against NaVi. They just had insane NiKo who was supported either by rare-fragger JKS or hunter. They were not good tactic-wise, they just got the kills.
NaVi played BAD, by bad meaning they had just awfull unlucky mistakes that are pretty easy to fix.
So i still see NaVi as one of the favourites in Katowice.