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MANCHESTER -- When was Riyad Mahrez's defining moment? In a first half where he and the rest of Manchester City's attack ascended to such heights as to leave Real Madrid players gawping in disbelief, was it the brilliant run and inch perfect cross that teed up Kevin De Bruyne to open the scoring after 93 seconds? Quite possibly. Even on its own, it would have been a moment worthy of lighting up a Champions League semifinal first leg that made moments of sheer excellence its stock trade. 

However if you had spent these 90 minutes watching Pep Guardiola and Pep Guardiola alone you would not have imagined this was Mahrez's most telling contribution. You would also have missed out on one of the great games that this competition has produced in recent years, but that is by the by for now. The Manchester City manager certainly reveled in his side taking an early grip on this semifinal with Real Madrid, one which they lead 4-3 ahead of their visit to the Santiago Bernabeu in eight days' time, but that delight was soon replaced by an apoplexy notable even for this most demonstrative of head coaches.

A right footed shot slammed into the side netting was the object of Guardiola's wrath, or to be more precise the pass that Mahrez had not made across goal to Phil Foden. 

In reality the best option might have been for Mahrez to open up his body, take the shot on his far stronger left foot and bend it into either corner. But as Guardiola himself acknowledged recently, it is altogether easier to tell players what they should do when through on goal if you are not the one with the ball at your feet.

"I would love when they are there [for them] to make the right decisions in small spaces, high tempo," Guardiola said after the brilliant 2-2 draw with Liverpool that City ought to have won. "Tomorrow I'm going to sit there on my laptop, show them the game and pause. Then the game will be stopped, nobody moves. 'How don't you pass the ball there?' But in football they move. You have to take a decision in the half second."

Much as even the best players cannot always be relied on to have clear heads in front of goal, so too the most shrewd of tacticians do not always keep their cool. This was the moment when the tie might have been killed off, no wonder it feels more decisive than any of the magnificent seven goals that the first leg delivered.

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Still in the afterglow of a win -- even one by a tighter margin than he might have liked -- Guardiola had cooled down sufficiently to instead be toasting the good moments. "We missed them but we created them," he said. "We were there all the time. 

"Football is football. We have to understand that sometimes,…, the result is the result."

The issue for Guardiola was ultimately that Mahrez was far from the only player to deal in dizzying peaks and vertiginous troughs. This was a night to leave you baffled by what a City player had done. In those half seconds they could amaze you or baffle you. But even then, you could not help but sense a couple of better decisions in the final third and this tie might have been dead.

This was a Madrid team waiting to be killed off. City were insistent on playing with their food. Paris Saint-Germain can attest that a two goal advantage can swiftly vanish if the mood takes Luka Modric and Karim Benzema, scorer of another two gawp-inducing goals. Equally there is nothing stopping any of City's superstar forwards from doing the same. After a thrilling seven-goal match the sense remains that this City side is a team who cannot quite control this tie in the same way they did against Paris Saint-Germain last season. This felt like a repeat of the harum scarum clashes with Liverpool (even though Carlo Ancelotti's side are not remotely at that level). An opponent prepared to throw numbers at City's rickety backline feels bound for some measure of joy.

It might not be quite as difficult for them to defend in the Santiago Bernabeu. Joao Cancelo will have served his one match ban and could take a spot on the right with Oleksandr Zinchenko having performed well against Rodrygo across the pitch. However, Cancelo is not quite as effective a recovery runner as Kyle Walker, whose lightning pace was never more missed than when Vinicius Junior exploded past Fernandinho, Road Runner against a beat cop one day away from retirement.

If Madrid can exploit the flanks again as they did tonight, if Benzema continues to defy the laws of expected goals, then City might just need to outshoot the La Liga leaders. That is where Guardiola might begin to worry. As Foden acknowledged "in these games, we need to take more chances." Though, even then it's worth remembering that being clinical is a funny old thing. For all that City seemed to miss chance after chance, they still out performed their expected goals tally of 2.37. It's a measure of just what City can do that in their best moments they blow away expectations while still leaving you feeling that there was more to be had.

Foden was not blameless even if he did score an excellent header after failing to pounce on the rebound from one of a string of Mahrez misses. Really though, it was the Algerian that typified this team in all its excesses and insufficiencies tonight. He would jink one way and another, gliding through the away defense as though they were not there. Then, with the hard part done, his shot would bend beyond the far post, Madrid's blushes spared.

For 90 percent of that move, and that game, he and City were excellent. But the final 10 percent tends to be what separates the winners from the field. And not for the first time it is there and there alone where the doubts are strongest.