Barcelona go into the Calderon with a goal advantage, but it's the slimmest of slim leads. Atletico got the all-important away goal, as the cliche goes, in a 2-1 loss at Camp Nou.
Barcelona have a built-in safety net. Any draw, or indeed a loss of 1 goal in which they score at least 2, and they go through. But there's a reason why, historically, teams that lose the first leg 2-1 away advance 49% of the time, according to numbers crunched by stat maestro Mister Chip.
A 1-0 home win for Atletico isn't just well within the realm of possibility, it's almost their specialty. A 2-1 scoreline, followed by a penalty shootout win, also seems like a likely route for Atletico. Barcelona have been awful at penalties; Atletico went 8-for-8 in a shootout against PSV Eindhoven.
But encouragingly for Cules, Barcelona have had Atleti's number in the last few encounters. Luis Enrique is 7-for-7 against Diego Simeone's men: that is, 7 wins in 7 matches, a ridiculous efficiency against such quality opposition.
And yet... it's hard to think this streak will go on forever, especially as the vast majority of those wins have been by a single goal. In fact, bar for the very first meeting between the managers in their respective reigns, when Barcelona won 3-1, it's been just the goal between the two of them.
Atletico don't concede many, so perhaps it's not surprising, but it does suggest that the gap in quality is very small indeed.
All this means is Barcelona must forget their slump in the league (3 matches without a win) as well as their win streak over Atletico (7 in a row) and focus on getting the result. It certainly won't be easy.
It's hard not to think Atleti's press will give Barcelona headaches early, but as always, the team must rely on particularly Sergio Busquets to get things going from the back. Then, obviously, it falls to the vaunted MSN trio - Messi, Suarez, and Neymar - to not only create chances but to relieve the defense by maintaining the ball and, when the time is right, helping out defensively.
Because of the tight competitiveness between the sides, the situation at 0-0 is quite precarious.
One can envision Atletico fancying their chances at 0-0 with say, 70 minutes gone, even if it wouldn't be an ideal situation for them, either. Score one, and Barcelona have only a few minutes to try to react. Simeone can then park the mother of all buses and knock the Blaugrana out.
That's why getting on the board first is so important for Barcelona. They would force Atletico to take chances, seeing as they will by then need 2 goals just to force extra time.
With maybe a chance to counter or two, Barcelona could put it well to bed then. A 2-0 or even 2-1 looks like goodnight for Atletico. Remember, 3-2 is not enough for Atletico thanks to the away goal.