Porto's hopes of reaching the UEFA Champions League knockout stages were severely dented as Austria Vienna claimed a surprise 1-1 away draw.
The Portuguese side secured a point courtesy of a 48th-minute header from Jackson Martinez, but failed to find a winner despite applying plenty of pressure.
Roman Kienast had given Vienna a shock lead in the 11th minute, capitalising on slack defending to fire home a low shot from long range for his side's first group-stage goal.
Vienna can no longer qualify for the last 16, while Porto must win at Atletico Madrid on matchday six to have a chance of overhauling Zenit St Petersburg and claiming second spot in Group G.
Should Zenit defeat Vienna in their final pool match, Porto will not be able to progress.
Home coach Paulo Fonseca made three changes from Saturday's 1-1 draw against Nacional, with midfield duo Lica and Steven Defour taking the places of Silvestre Varela and Hector Herrera.
Nenad Bjelica unsurprisingly opted to stick with the same Vienna line-up that recorded a 5-0 win over Wacker Innsbruck at the weekend.
The visitors wasted little time in finding the target again as Kienast shocked the home crowd.
The 29-year-old forward picked up a loose ball out of defence with yards of space around him, before keeping his composure to fire neatly past goalkeeper Helton.
Heinz Lindner was soon called into action at the other end, tipping Martinez’s header over the crossbar.
The Austria goalkeeper came under increasing pressure as the half wore on, with Vienna penned further and further back into their own half, but the hosts struggled to find an effective final ball and ended the half behind.
Porto appeared to be well served by the interval and it was not long before Martinez rose high to score after Eliaquim Mangala had nodded back across goal.
The Colombian had a fine chance to notch the winner in the 64th minute, but after getting mixed up with strike partner Varela, who was introduced at half-time, he saw a deflected effort land on the roof of the net.
As full-time approached, the hosts continued to turn the screw but just could not find a way through, with Martinez’s header eight minutes before time also sailing over.
Porto now boast five points from as many group matches and must hope Vienna are able to prevent Zenit from taking maximum points on December 11.
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