In Thomas Tuchel’s first senior football job at Mainz, the German coach was sitting on the team bus watching a documentary about Pep Guardiola when he suddenly paused it. A graph showing Barcelona’s pass patterns had come up on screen.

“To anyone else, it just looked like a knitting weave,” former Schalke official Christian Heidel says. To Tuchel, it was a treasure trove of information. The screen stayed paused for two hours. That was how long Tuchel spent studying the graph.

It is little wonder Guardiola insisted on going along when he found out Bayern Munich technical director Michael Reschke was meeting Tuchel for dinner back in 2014. Guardiola had also seen something in the young German coach, who was then on sabbatical.

“Pep doesn’t just want to meet anyone in football,” Reschke says. “He had a feeling about Thomas.”

“This is an interesting young coach,” Guardiola enthused to Reschke. “He has an idea.”

When they finally met, there were some opening formalities, with both men asking about the other’s family. After both declared them to be “fine”, it was just straight into football. For hours. And into minute detail.

Tuchel had naturally travelled to Camp Nou to watch Guardiola’s Barcelona five years beforehand, and began to ask him about exact minutes of play, as well as specific decisions the Catalan made.

Of particular interest was how Cesc Fabregas’ signing changed the team, and Leo Messi’s role. Guardiola was only too willing to get very deep into it.

The conversation developed into an elaborate and enthusiastic discussion about how you create space on a pitch in given situations. It went down to milliseconds of play and millimetres of pitch.

“I was just sitting there in disbelief,” Reschke says. “It was amazing, like (chess players) Garry Kasparov and Anatoly Karpov.”

It was a meeting of minds. They just happened to be two of the most obsessive minds in the modern game.

Many people who know Guardiola say the only time he actually switches off from football is when he insists on turning off his phone to play golf.

The course is the one place he is sufficiently immersed in something else to not think about the game. Beyond that, he’s always on, right down to badgering the canteen staff at Manchester City’s training complex. Tuchel appears to have more interests – given that he has been a cocktail maker, and a model – but he is much the same.

“He thinks about football 24-7,” Heidel says. “His work day is not normal. We are talking about absolute football fanatics.”

This is about more than the idiosyncrasies of two super-obsessive men, though. The fact they meet again in club football’s greatest fixture illustrates how this intense mindset has gradually taken over the elite end of the game.

It isn’t quite revenge of the nerds, to use Tuchel’s own word, but it is a reflection of how a higher level of thinking generally leads to a higher level of performance.

Inevitably, some managers will beat them in an individual match – or even over a brief period of time – but if they can’t match them in preparing for every square metre of the pitch, the gaps only grow greater over time. Put simply, less is left to chance. Such intensity of thinking leads to greater innovation, too.

A deeper comprehension of the game’s intricacies fosters the understanding to realise something different and the confidence to try it.

“These coaches are able to read the game on a higher level,” Reschke says. “They see things you and I will not see.”

This can be witnessed in the unique spectacle of Guardiola’s teams.

Wayne Rooney recently told Jamie Carragher there were only two times when he was confused about what he had to do on a pitch.

One was against Barcelona in the 2011 Champions League final, one of Guardiola’s career performances.

The other was against Manchester City in the 100-point 2017-’18 campaign, one of Guardiola’s landmark seasons.

It was as if the Catalan’s teams were always playing several steps ahead, putting in passes that would open you three moves down the line.

Heidel points out how opposition sides in the Bundesliga “hated” facing Tuchel’s sides.

You might even call it the NFL-isation of football. The game is just so much more rigorously prepared than even a decade ago. Bill Shankly’s maxim is no longer true. Football is no longer a simple game complicated by idiots, but an increasingly complex game, enriched by geniuses.

Much of this comes from the tactical evolution first sparked by Guardiola at Barcelona.

“I think, 10 years ago, Pep created a new idea of football,” Reschke argues. “It was the main influence on football in the last 30 years. All of the intelligent young coaches – Julian Nagelsmann, Thomas Tuchel – are influenced by him. You can see it in England, too.”

Guardiola’s possession-pressing game was gradually complemented by advances in sports science and analytics. They caused a huge leap forward in team preparation. All of this combined to ensure football became infinitely more co-ordinated, from pressing to attacking.

For most of football’s modern history, defending was necessarily predictable and organised, while forward play was free-form. This is one other element that has been turned on its head by coaches like Guardiola.

Fabregas recently told ‘The Wall Street Journal’ he realised the true extent of this under Antonio Conte at Chelsea in 2016.

“It was more like, ‘You have to do what I want you to do. You will do this, and we will repeat it over and over again, for months and months, until you get it right.’ And we will play like a mechanized robot thing. You can play with your eyes closed. The ball comes to you and you know what you have to do, because your team-mate will be exactly at the right time at the right place.”

Tuchel isn’t identical, but he is along the same path. The German spent one Chelsea press conference speaking about how his approach is to put in place a structure that eventually puts players in the position to show “their creativity”.

“It’s just to give possibility, and from the possibilities it is the free choice of the players,” Tuchel explained.

This is what Guardiola has also taken on, as his own approach has evolved since Barcelona.

“Football has developed,” Reschke concurs. “It is now like American football, where you have special coaches. OK, not exactly the same, but closer.”

This is why Tuchel starts screaming at players when they are a metre out of position. It can make the difference between, say, one-on-one defending working dependably and proving a disaster. 

This is also a man who has been known to get down on his knees ahead of training sessions, not just to measure the grass but – according to Heidel – “to sniff” it. Tuchel’s ideal length is 2.7cm, to better facilitate fast passing.

It can be a little hard to square that kind of obsessiveness with another side of the game, where successful managers like Zinedine Zidane talk so vaguely about “enjoying the ball”.

“Football is played by humans so there are a lot of factors, but it is incredible that clubs like Juventus and Chelsea have just gone for former players like Andrea Pirlo or Frank Lampard,” one official who has worked at a top club argues. “Who knows if they will even be good? The game has moved beyond talking about mentality or experience.”

Much of this becomes apparent on the training ground. That, to a far greater degree than ever, is where games are really won and lost. That is the only place players can really learn and internalise the intricate approaches set out by coaches like Tuchel and Guardiola. It is why many in the game believe Chelsea can be particularly dangerous next season, as Tuchel has proper time to prepare.

“What we try to implement in training is the mix of respecting your zone,” Tuchel explained in February, “and to have a structure and then the freedom for the creativity and the intuition of players. That is a constant mix.”

It is why Tuchel is constantly “demanding”, according to captain Cesar Azpilicueta.

“Training is very specific,” he says. “His focus is on every action, when a player is out of position.”

This is one area, nevertheless, where Tuchel is said to be more “relaxed” than Guardiola. The Catalan’s sessions are said to be innovative, but what really elevates them is the manic intensity.

One drill, for example, involves three players standing in a column, but all moving along rows at the same time while passing to each other. The idea is that the ball always keeps moving between them at the same pace. That’s fine when that pace is relatively relaxed. That is not what Guardiola demands.

There are common refrains that ring around the training ground.

“No, no, no!”

“Always fast, the ball never stops!”

“Joshua Kimmich told me that, after one year of Pep, he would be nervous before every training session,” Reschke reveals. “He knows he has to be focused. There is no idle talk.

“When you train with Pep, everything is top level. He shows you how to pass, how to stand when you receive the pass. The difference is the detail.”

These two together go deeper than almost anyone in football. That will now be seen by the whole world in Porto, where the scale of the occasion will only cause Tuchel and Guardiola to fixate on ever finer detail.

It will decide this season’s Champions League but has already defined the future of the game.