While Porto will kick off their 18th Champions League campaign in the Ernst Happel Stadium, their opponents squeezed through to the group stage for the first time after a tense play-off win over Dinamo Zagreb.
And the form of both sides could hardly be in greater contrast heading into the game, with Porto sitting on top of the Portuguese Liga with maximum points from four games, while Vienna are fifth in the Austrian Bundesliga and without a win in their last two.
But Fonseca said: "We are not playing at home and we are facing opponents who have reached the Champions League on great merit. We do not have an easy task.
"The importance of winning this game has more to do with the fact that it is important to start off well in the group stage. We can only prove that we are favourites with what we do on the field."
Fonseca does not expect to make any major team changes for the tie in Austria as his side look to get off to a dominant start in a group that also includes Atletico Madrid and Zenit St Petersburg.
Fonseca's opposite number Nenad Bjelica has admitted his side face a major task against opponents he classes as "incredibly strong".
His side have not had a good time of late with a home draw against Wacker Innsbruck followed on Saturday by an embarrassing 3-2 home defeat to Grodig.
Bjelica said: "Of course we are the underdogs. They are an incredibly strong team. We will go out to get three points just as in any other game, but the chances of winning against such opponents is naturally small."