The 40-year-old manager has highlighted the importance of this week's group stage game as his side look to put breathing space between themselves and their Russian opponents
The Portuguese champions hold a two-point lead over the Russian giants ahead of the game and Fonseca feels Tuesday's encounter will be vital for both sides.
"It might be a very important game for us, but it is also very important for Zenit," the Porto coach said at a press conference.
"Every team has the possibility of going through in our group. Our ambition is always the same, no matter the opponent we are facing, we always want to win."
Porto will come up against former player Hulk on Tuesday, with the forward scoring 54 goals in 99 league appearances in his time with the Portuguese club, but Fonseca has claimed his side do not need a special plan to cope with the Brazil international.
"He was one of the best at Porto, but now that means nothing. We’ll play against him exactly the same as we do against other opponents.
"The team's getting better and better every day, but it's really the result, not the quality of the football, that will come first."