The Primeira Liga side need to beat the Group G leaders in Spain and hope Zenit St Petersburg drop points against Austria Vienna if they are to reach the knockout stage.
Atletico ran out 2-1 winners in the reverse fixture but Fonseca does not want his side to adopt a gung-ho approach in the Spanish capital.
"Atletico was a powerful team when they played in Porto and now they are even more so," Fonseca said.
"They are having a very good season with one loss and only a draw at home but Porto come only with the aim of winning because there is no other option.
"We expect to win but we must show a balanced game during the whole match. I am completely sure we can win and we will see what happens with Zenit's match but we cannot do anything about that."
Fonseca is sweating on the fitness of midfielder Lucho Gonzalez, who has scored twice in the group stage.
"We will wait to see how Lucho Gonzalez evolves and if he can play," he added.
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