Can Atlético Madrid last the pace? Will Pep Guardiola's tinkering harm Bayern Munich's hopes? Does José Mourinho have the right gameplan and will Real Madrid's forward line be too strong for everyone? Here we analyse all four teams
How are they not exhausted yet? Atlético benefited in Europe two years ago when they faced a shattered Athletic Bilbao in the Europa League final in Bucharest, sweeping them aside 3-0, and the fear all season has been that they would suffer a collapse similar to that suffered by Marcelo Bielsa's side. The intensity isn't quite so high, but it is high enough and, with a limited squad, fatigue is a realistic concern. Until or unless it hits, though, Atlético plough on, a defensively solid side that presses intelligently, eschewing the all-out frenzy of bielsisme for something more carefully targeted – which should help to preserve energy.
The basic shape is usually 4-4-2, often becoming 4-2-2-2, with Gabi and Tiago Mendes sitting deep in central midfield and a flexible front four – usually led by the muscular and prolific Diego Costa, although Atlético's performance without him Wednesday showed they are not reliant on him. The real strength, though, is probably the solidity of the back four plus Thibaut Courtois (although it seems the Belgian goalkeeper would not play if Atlético faced Chelsea so as to avoid an appearance-against-his-owners fee that was written into his loan agreement). Diego Godín has had a superb season alongside João Miranda at the heart of the defence, Juanfran is a calm and experienced right-back and Filipe is tireless on the left. In 10 Champions League games this season, Atlético have conceded only five times.
There is no squad in Europe more packed with talented midfielders and forwards than Bayern, no coach who sees the game quite like Pep Guardiola. To watch him in action on the touchline is to see a study in micromanagement, forever tweaking and adjusting, playing some extraordinary game of dynamic chess in which only he can see the board. If the opposition don't push men forward, he feels no need to leave men back, the result being the W-W shape he has employed at times this season, including against Manchester United on Wednesday. Critics suggest he overcomplicates the game needlessly – and the danger of shifting too many men up the pitch, quite apart from the defensive risks, is the potentially loss of verticality: if there aren't players making bursts from deep, it can be difficult to generate the sort of forward thrust necessary to puncture a blanket defence. It's certainly true that the high line Bayern play can leave them vulnerable to pace, but then the diligence of the midfield pressing tends to be enough to alleviate that. They have, after all, won 37 of 46 games under Guardiola, and a draw and two of the three defeats have come in tournaments that had already been won or after qualification for the next stage had been secured.
Not only is Guardiola forever tweaking but this is a side that gives him a vast array of options: Bayern can play a far greater variety of styles than his Barcelona could. Possession, of course, is king and proactivity and ball retention are the central tenets. Within that, though, there is, to take only the most obvious example, the option of playing with a false 9 in Thomas Müller or Mario Götze or of going long to Mario Mandzukic. There may be nobody with the individual game-changing brilliance of Lionel Messi, but Bayern have plenty of ways of winning.
It may not have seemed like it at the end of the first leg of the quarter-final, when Javier Pastore wriggled through three half-challenges to score what threatened to be a decisive third goal for Paris Saint-Germain, but Chelsea vie with Atlético for the best defence of the four sides left in the competition. They also have, in José Mourinho, a manager who has that most underrated and most difficult to define of qualities, the capacity to take a fifty-fifty situation and make sure he comes out on top. Tuesday's comeback was about the players, of course, it was about indefatigability of Branislav Ivanovic, César Azpilicueta, David Luiz and Willian, Petr Cech's concentration and reflexes, the scurrying and imagination of Oscar, the general solidity of Gary Cahill and John Terry, but it was also about the preparation, tactical and mental, of Mourinho, and about his capacity to transmit his ferocious will to win to his players.
Mourinho knows his side don't have the attacking strength to take on an opponent at this level in an open fight but he relishes the opportunity to execute a gameplan to prevent opponents from playing and, in Eden Hazard, Willian, André Schürrle and Oscar, he has the ideal creative midfielders to play on the counter, sitting deep and springing forward on the counterattack at pace.
What had been apparent in the clásico was just as evident against Borussia Dortmund in the quarter-final. Across their forward line, this Real Madrid side are as dynamic as any team probably ever, Cristiano Ronaldo and Gareth Bale offering a very modern blend of technical ability, pace and explosive strength, while having in Karim Benzema a forward whose willingness to drop deep makes him an ideal foil. Few sides would fancy their chances of holding off Real Madrid.
The problem is at the other end. Although the midfield three of Xabi Alonso, Luka Modric and Ángel di María are capable of controlling possession rather better than Madrid sides have in recent years, the deficiencies behind them mean they can still be overwhelmed. The lack of width is inherent in the 4-3-3 but it is exacerbated by the lack of tracking done by Ronaldo (and to a lesser extent Bale). For a full-back to leave either and surge forwards is an enormous gamble, of course, and Di María works as manfully to fill the gap as anybody could reasonably expect, but any full-back who does dare venture into the Madrid half is likely to find himself almost untended, particularly if Di María is occupied dealing with a midfielder. Add to that Marcelo's shortcomings as a defender and the Madrid left, while a great strength from an attacking point of view, is a vulnerability.