Against Barcelona, the hardest lesson every opponent learns is that what comes to define their performance is the few precious seconds when their concentration slips, the gaps open up and those famous striped shirts swarm through, like raiders at the castle gate.
It was in those few moments of weakness last night, when Manchester City allowed themselves to be found off-guard and badly mismatched in defence, that the balance of the tie shifted.
This current Barcelona, like its modern predecessors, is a pitiless opponent and, as is their way, they seized upon City's biggest error of the night and punished them severely.
It was Lionel Messi who drew the foul that saw Martin Demichelis sent off on 53 minutes, and it was Messi who dispatched the penalty, although there was no doubt that contact began outside the box.
In most other respects, the Argentine was corralled and controlled by City, even subsequent to the goal and with just ten men on the field. But almost good-enough is never enough against a side this ruthless.
That they managed to keep the scoreline at just one goal for so long was an achievement, given that Messi and his men had more than half an hour potentially to put the tie out of sight, but the second was a dire outcome for City.
They had made it all the way to the last minute of normal time when Dani Alves scored the goal that surely puts this tie out of reach for them at the Nou Camp in two weeks' time.
It is not the first time that an English opponent has found itself trying to fight off Barcelona with just ten men. It was the same for Arsenal in the 2006 Champions League final with Jens Lehmann's red card and then again in 2011 when Robin Van Persie was sent off.
Chelsea endured the same fate with John Terry two seasons ago but got the draw that saw them through.
There is no margin for error when challenging the likes of Messi, and Demichelis was not even close.
Manuel Pellegrini waited to give Swedish referee Jonas Eriksson a piece of his mind at the end of the game over the penalty. With 10 men and one-goal down, City were still in it, and fought well, but this is now a monumental task.
For the first 20 minutes, Barcelona gave their opponents the kind of examination that only Barcelona can. When they keep the ball in that fashion, it behoves the opposition to hold their nerve, stay tight and attempt to keep the swarms of Barça shirts a safe distance from the goal.
City did that, and they did it well. The temptation must be to panic when such long periods pass without a decent spell of possession, and when the rare successful tackles just seem to give the ball back.
This is Barcelona's greatest trick, keeping the passes ticking over while they decide where the other side's weak spot is located. But City were ready for it, and handled it as best as could be expected.
For that period of possession, City did not permit a decent strike on goal from Lionel Messi, Xavi or anyone else for that matter.
There were efforts later in the first half, the best from Xavi that forced a fine save from Joe Hart, but the best chances came for the home side.
It was a Yaya Toure run that took him into Barcelona territory and a pass right to David Silva that seemed to break the spell. In the centre, Alvaro Negredo was nudged as he jumped for the ball but at the very least the Barcelona defence had been placed under pressure.
Within two minutes, Silva had played in Negredo but he went too wide to be able to hit the target.
From then on, City never quite found themselves under the cosh as they had before. The home side looked dangerous from set-pieces, rightly putting everything into an aspect of the game that Barcelona have struggled with all season.
With Carles Puyol missing again, there was simply not enough height in the Barça defence to deal with all City's candidates for a firm header.
Vincent Kompany epitomised that aggression on 25 minutes but was adjudged to have fouled Victor Valdes.
The supply route to Messi was largely successfully disrupted and his first shot on the half-hour went over. Xavi's followed two minutes later.
Then, seven minutes before the break, came a let-off for City. Aleksandar Kolarov, playing on the left side of midfield, launched himself at Dani Alves, himself already booked, with studs raised. A more zealous referee might have reached for the red card but he got away with a booking.
There was one more effort, a minute before half-time, when Negredo headed wide. He was under pressure but might have done better given the precision of Jesus Navas's ball into him. It was that way City's best chance of a breakthrough lay.
The blow came with eight minutes of the second half played, with an innocuous beginning out on City's right.
Navas looked like he might have gone down too easily under a challenge from Sergio Busquets.
The Swedish official had looked in the mood to give every foul he could, but he waved play on. The ball reached Andres Iniesta who looked up and saw exactly what he wanted.
That was Messi with Demichelis at his shoulder and no one else between him and the goal.
He gathered the pass beautifully into his stride, accelerated away from his fellow Argentine while the stadium held its breath for the tackle.
When it came, it was as bad as the home fans could have imagined: a clip of Messi's ankle with the ball nowhere near and the inevitable tumble.
There was no questioning the red card for the denial of an obvious goalscoring opportunity. The only decision facing referee Eriksson was whether to give a penalty or not. Initial contact was outside the area. Demichelis walked off.
Messi dispatched his penalty down the centre when Hart committed himself early to the left.
Pellegrini reorganised with Joleon Lescott coming on for Kolarov; Silva moving back to left midfield, and Samir Nasri a straight exchange for Navas. With the emphasis then on keeping Barcelona out, City acquitted themselves extremely well in the circumstances.
Victor Valdes had to save a shot from Silva. Gael Clichy should have done better when he got free down the left.
Then a slip by Lescott allowed a one-two between substitute Neymar and Alves to get the full-back in on goal and he put his shot through Hart's legs. The game was over, and the tie is all but gone for City.