Porto coach Paulo Fonseca has no illusions as to the magnitude of Tuesday night's Group G match against Zenit St Petersburg as his team look to respond to the defeat by Atletico Madrid last time out.
The Portuguese champions were a goal ahead against Atletico at the start of the month before the La Liga side turned the game around with Diego Godin equalising before Arda Turan scored the winner four minutes from time.
It leaves Porto in second with three points from two games, following their victory over Austria Vienna, three behind Atletico who will start to put clear daylight between them and the rest of the group if they beat the Austrians and the Portuguese fail to win.
The match is vital for Zenit too who so far have just one point from their goalless draw with Austria Vienna.
Fonseca told a press conference: "It might be a very important game for us, but it is also very important for Zenit.
"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.
"With Atletico we had some difficulties, but those only made us stronger and we have learned from them.
"I am happy with the past two weeks despite the absence of important players in the national teams, because our defence was complete during this period, which allowed us to get our processes right and improve our defensive side of the game."
Zenit coach Luciano Spalletti still has confidence in his players despite them being third at the moment and a positive result on Tuesday would be invaluable given the teams meet again in Russia in a fortnight's time.
Spalletti told a press conference: "My guys know that they are entirely capable of advancing to the play-off stage.
"I prefer to think about still getting first place in our group. We are taking Porto very seriously."
Alex Witsel is not available following a contentious red card last time out but the coach refused to dwell on that.
He said: "I'm not the kind of guy who's going to cry about not having a player like Witsel available. We have what we need to win.
"The absence of Witsel had a bigger influence in our previous match, when we lost him for the second half versus Austria.
"We have everything we need to play at the highest level."