Isco scored in Monday's dramatic stalemate in Kaliningrad to earn the man-of-the-match award and his country top place in Group B, on goals scored, from Portugal.
Portugal face Uruguay in the next round.
"It's going to be a tough game, we're playing the hosts, the entire country will be behind them. They play excellent football," said Isco. "We played a friendly against them before the World Cup, we can't make any mistakes."
The Real Madrid star added: "We're happy we've finished top of the group, that's what we wanted. Maybe we wanted to feel differently after the match but we're happy to go on. Now we are facing the moment of truth. I have complete faith in this team."
Fernando Hierro, who took over as coach hours before the World Cup began after Julen Lopetegui's sacking, said: "We know full well Morocco were going to fight, the entire group was a tough one, history bears that out. The first conclusion I draw is that despite all these challenges we ended on top. I'm looking at the silver lining. Obviously we can improve. Five goals in three matches is not the way forward. The players are professional, they understand what the situation is."
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