Celtic's Europa League hopes are hanging in the balance following their narrow defeat by Atletico Madrid in their penultimate Group I game at Parkhead.
An Arda Turan strike in the 29th minute was enough to secure the Spanish club qualification for the knockout stages with 10 points.
The Hoops have to beat Udinese in their last group fixture in Italy on December 15 to leapfrog the Italian club into second place and, with a poor away record in Europe, their task is uphill to say the least.
Celtic had more than held their own against Atletico until the Turkey international picked up a Daniel Majstorovic clearance and fired back in past Fraser Forster.
Substitute Gary Hooper had the ball in the net after the break, only to see it chalked off with an Anthony Stokes handball apparently the reason.
Atletico were slick on the counter, but it was from a corner that the Spanish club took the lead through Arda. Diego's ball from the right was headed clear by Daniel Majstorovic only for the Turkey international to drive powerfully back in from 20 yards.
Atletico started the second half as brightly as they had begun the first period, and within two minutes of the restart Gabi had surged forward before sending a shot wide of the target from the edge of the box.
At the other end, moments later, Stokes linked up with Samaras before the Irishman's low drive was saved by Courtois down at his right-hand post.
Stokes was replaced by Scott Brown in the 75th minute and the home side began pushing for the leveller, which, ultimately, failed to arrive.
Meanwhile, Tottenham were left with a very slim chance of making the second round after they were beaten 2-1 by 10-man PAOK Salonika on a controversial night at White Hart Lane.
Sloppy marking from Sebastien Bassong allowed Dimitris Salpingidis and Stefanos Athanasiadis to give the Greeks a 2-0 lead after 13 minutes, but Spurs pulled one back with a Luka Modric penalty after Konstantinos Stafylidis was dismissed for handball on the line.
Tottenham thought they had equalised through Jermain Defoe in the second half, but referee Bas Nijhuis controversially disallowed the striker's goal for a foul on Pablo Contreras.
Birmingham paid the price for Nikola Zigic's early saved penalty as a narrow defeat in Braga left them facing an uphill battle in their bid to progress to the knockout stages.
The Serbian saw his 11th-minute spot-kick saved by goalkeeper Quim and the Portuguese hosts went on to claim victory thanks to a touch of City misfortune as Hugo Viana's 50th-minute strike took a huge deflection off Curtis Davies.
The result saw Braga qualify for the last 32 from a tight Group H, while Club Brugge's dramatic fightback to claim a 4-3 victory over bottom club Maribor also sent them three points clear of City.