Did you also consider the age of this players. The Croatia players are older than the Japan players and that is why the Japan players are faster and they cam return to help the defence when needed. This the only fair fixture I have seen in the knockout stage of the World Cup. Croatia vs Japan is a far fixture.
The matches in the round of 16 were determined by the results that were in each group and that was clearly completely fair because it was impossible to engineer it in such a way. And I think the match between Croatia vs Japan is a match that will look exciting even though the Croatian team will probably try to have more possession of the ball from the start.
Of course their meeting in this round is based on their results in the group stages. His Croats have more experience, but Japan came with more motivation and they have very high speed. This match will be interesting because both countries have their respective advantages.
In the other match between Brazil vs South Korea, it will be a match that places Brazil as the favourite. How could it not be, Brazil has a luxurious squad of both their core players and players who sit on the bench.