The Champions League is widely considered the toughest club footballing competition in the world. And for good reason.
Since the inception of the tournament in 1992, only five sides have managed to defy the odds by winning all of their group matches.
The famous five will turn into the sensational six if Atletico Madrid can win its final fixture in the group stage away to German giant Bayern Munich - a monumental task.
Atletico has already qualified for the knockout phase, but the passion for the match is very much alive inside manager Diego Simeone.
"The importance of this Tuesday's match is marked by the importance of the rival," Simeone said at a press conference Monday, according to the club's website. "Sometimes, pride counts much more than the points."
Here's a look at the other five teams who managed this incredible run, and how they fared in the latter stages of the tournament:
AC Milan (1992-93)
AC Milan is the only side to make it to the final after winning every game in the group stage.
After winning 4-0 against IFK Goteborg in Milan on opening night, the club won by a single goal in each of the next four matches, before taking the final game of the group stage 2-0 against PSV Eindhoven at the Stadio Giuseppe Meazza in Italy.
The final took place at the Olympiastadion in Munich, where Milan lost 1-0 to a Marseille side led by Belgian manager Raymond Goethals.
Paris Saint-Germain (1994-95)
PSG started its 1994-95 Champions League campaign in style, beating Bayern 2-0 in France thanks to goals from the legendary George Weah and Daniel Bravo.
Les Parisiens went on to defeat Spartak Moscow and Dynamo Kyiv home and away, securing a spot in the tournament's quarter-final.
In the quarter-final, they squared off with Barcelona in a clash of the titans that ended in success for the team from France, which won 3-2 on aggregate thanks to a 2-1 victory at home in the second leg.
Unfortunately, PSG was booted from the competition by Italian behemoth Milan after losing 3-0 on aggregate at the semi-final stage.
Spartak Moscow (1995-96)
Unlike PSG in 1994-95, Spartak did not open its campaign in the most convincing manner.
The Russian side spluttered over the line with a 1-0 win in England against the high-flying Blackburn Rovers and a 2-1 win against minnow Legia Warsaw.
After the initial couple of games, Spartak found its true form against Rosenborg on matchdays three and four, winning the matches 4-2 and 4-1, respectively, before returning home to crush Blackburn 3-0.
All of the group-stage magic seemingly wore off, though, as the club proceeded to bow out to French side Nantes, losing 4-2 on aggregate after it lost the first game 2-0 away from home.
Barcelona (2002-03)
Current manager Luis Enrique was the hero for Barca in its opening night fixture, scoring against Club Brugge at home to spark his team to a run of nine consecutive victories in the Champions League.
The sequence of wins included trips to Galatasaray and Lokomotiv Moscow, plus the return fixture against Brugge.
Los Blaugrana eventually fell victim to Juventus in the last eight of the competition, losing 3-2 on aggregate, with Marcelo Zalayeta scoring the winning goal in extra time at the Camp Nou.
Real Madrid (2011-12)
Under the guidance of media sensation Jose Mourinho, Madrid scored 19 goals and only conceded two during its Champions League campaign in 2011-12.
Madrid was matched in a group with Dinamo Zagreb, Ajax, and Lyon.
After an unimpressive victory away from home against Zagreb, Los Blancos found their finishing boots and went on to crush Ajax 3-0 and Lyon 4-0 at the Santiago Bernabeu.
They made it through to the semi-final after beating Cyprus' APOEL over a two-legged affair in the quarter-final.
The semi-final could not be decided through open play, and Bayern won the game after penalties were needed to separate the two titans.
Real Madrid (2014-15)
After winning the Champions League for the 10th time in 2014, Madrid kick-started its next campaign with a bang.
Los Blancos walked over Swiss champion Basel, winning 5-1 at the Bernabeu on opening night.
Aarlos Ancelotti's side did not take its foot off the gas, following up the first win with impressive victories away to Ludogorets and Liverpool, and ending the group stage with another one-sided fixture that ended 4-0 against Ludogorets in Spain.
Just like the invincible team fielded in 2011-12, the semi-final proved to be the final stepping stone for a Madrid side that won the competition less than 12 months earlier.
Madrid met Juventus in the last four and lost 3-2 on aggregate due to a goal from Alvaro Morata in his return to the Bernabeu against his old side.
Nice welcome back present, Alvaro.
