Artmedia Bratislava pulled off a shock 3-2 away win over Porto to throw Group H, which also features Rangers and Inter Milan, wide open. The home side went two goals up through a Luis Gonzalez header and a Diego shot before Peter Petras pulled a goal back in first-half injury time.
The Slovaks pulled level through Jan Kozak before Balasz Borbely grabbed the winner from close range.
"We're from a small city but we have a big heart," said coach Vladimir Weiss.
"We knew Porto's wingers attacked a lot so our strategy was to take advantage of it, working the diagonals. Fortunately the strategy worked.
"Our first goal picked up the team's spirit and got them to believe in coming back. After that, in the second half, we scored two more goals and from then on we didn't let the win get away."
The game marked Porto's 112th birthday celebrations, which were somewhat spoiled by the result.
"I'm responsible and I feel ashamed Porto have lost on this birthday," said Porto coach Co Adriaanse.
"I apologise to the supporters because I invited them to come here.
"We were well organised on paper but it's another thing to
do it on the pitch.
Asked if he would change Porto's style in the next group match against Inter Milan, he said: "I'm not a coach that changes quickly.
"We were playing very well, I think we were dominating the game. Normally you win those games, why should you change your style?".