The Portuguese champions host an in-form Atletico Madrid side at the Estadio do Dragao on Tuesday, with the Spanish team boasting a 100 per cent domestic record this term.
While Porto got their European campaign up and running with an away win over Austria Vienna, Fonseca maintains the results from their next two home games against Atletico and Zenit St Petersburg will be vital in their quest to progress.
"We know the importance of home games. It is important for us to have a good record here and that means winning the two meetings," he said.
Atletico maintained their perfect start to the Liga season with a 1-0 derby win over Real Madrid at the Bernabeu on Saturday.
And Fonseca believes Atletico's domestic form, coupled with their win over Zenit in their Champions League opener, makes Diego Simeone's side tough opposition.
"We will face a very strong team, moralised in the light of recent results. We know the difficulties," he continued.
"All games are tests and have their degree of difficulty. We know we will face a very strong team. We will face difficulties that we have not had in our league."
However, defender Nicolas Otamendi feels that while Atletico are in good form, his team-mates can hold their own against them.
"Atletico Madrid is fighting with Real Madrid and Barcelona for the lead in Spain, but we are in the lead here in Portugal," he said.
"We will do anything to avoid conceding goals; from there, from the midfield to the attack, we want to have a good performance.
"At Estadio do Dragao, we will have to have a good game. Each team has its own style of play and we, because we play at home, feel the pressure to win the match, to have a good performance and to be well tactically."
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