Diego Simeone's tactical changes helped Atletico Madrid wrestle back control against Arsenal at the Metropolitano Stadium. His switch to a back five at half-time confounded the visitors. But his analysis of the game focused on another factor.
"I think you could see some of the fatigue they've been building up from so many matches, so much pressure, and the responsibility of having to win the Premier League and the Champions League," said the Atletico boss in his post-match press conference.
"All of that adds up. And in the second half, we started to play better, especially in the final third. We created more chances. We just couldn't get the goal to give us the edge."
Clearly, he had factored that physical and mental fatigue into his plan for the game. The Viktor Gyokeres penalty forced the issue, of course. But the difference in Atletico's intensity after half-time was stark. They set out to overwhelm their tired opponent.
If Arsenal looked in danger of sinking, maybe it is understandable. Their total of 57 games played this season is the highest of any side across Europe's major leagues and there is another hugely significant one to come against Fulham on Saturday.
Atletico have played the second-most, at 55. But fourth in LaLiga, safely in the Champions League spots though well short of a title challenge, the significance of their league campaign has long faded. They have been able to conserve energy.
Arsenal, of course, wouldn't trade positions. But conserving energy has not been an option for them. Instead, they have been pushed to their limits. In David Raya, Martin Zubimendi and Declan Rice, they have three players who have hit 4,000 minutes.
The sheer effort they have put into the campaign contributed to Mikel Arteta's fury at their overturned penalty.
"When you have fought so hard for nine and a half months to be in this position, that's another goal that changes the tie completely," he said afterwards. "It can't happen. I'm sorry. We put so much on it. So, so, so much on it. This cannot happen."
That penalty, had it stood, might have allowed Arsenal to secure a famous victory. But it is thanks in large part to their warrior spirit that they avoided defeat. Atletico's pressure was relentless at the start of the second half. Arsenal, though, stood up to the test, ensuring the Julian Alvarez penalty was the only breach.
Their determination to hold out was summed up by Piero Hincapie, who won every one of the five duels he contested and made nearly as many interceptions as the rest of the entire team. It was also evident in William Saliba and Gabriel, who made more than twice as many clearances as all the other players on the pitch combined.
It could, in fact, be seen across the team. When Ademola Lookman found himself free on the left-hand side of the box in the first half, it was Noni Madueke who got back to intervene. Gabriel Martinelli showed the same defensive diligence on the other flank.
"I'm very proud," added Arteta. "I said that to the boys, the way we handled the context for nine-and-a-half months is remarkable. I really value what they've done because I've seen some of the best teams in the world here fall apart and concede three and four."
The nine-goal thriller between Paris Saint-Germain and Bayern Munich highlighted the attacking potency of the other semi-finalists. But Arsenal defensive prowess is their edge. They have only conceded six goals in 13 Champions League games this season.
It is reasonable to wonder whether they fall back on that defence too readily. But Arteta would argue it is mostly done out of necessity, either in response to what knockout games such as this one demand, or to the injury issues that have affected so many of his attacking players over the course of the campaign.
On that front, they can take encouragement from the impact of their substitutes against Atletico Madrid.
Kai Havertz and Jurrien Timber remain absent but Bukayo Saka, Eberechi Eze, Leandro Trossard, Gabriel Jesus and Cristhian Mosquera all played a part in helping Arsenal regain control of the game following their introductions in the closing stages.
Their contributions had Simeone shaking his head when asked if the intensity of Arsenal's schedule makes Atletico favourites to advance from next week's second leg.
"No, no," he said. "Did you see the five players they brought on? Each time I said, 'Really?' Yes, really, because they are very good. They have so much depth to be able to compete in the way they are. Otherwise they wouldn't be, would they?
"They've won 10 matches in the Champions League, drawn three, and haven't lost a single one. In the Premier League, they have been fighting to win it since the very first day of the season. We are playing against a great team."
Arsenal need this epic season to end with silverware if they are to be considered truly great. But first there is Fulham. Another quick turnaround, from one vitally important game to the next, and another battle against fatigue as well as their opponent.
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