Football is complicated, even if it is a simple game. Almost every game should be approached with confidence, but you have to be acutely aware that you cannot trip on that invisible line and fall embarrassingly into the dangers of overconfidence. Do you know precisely where that line is drawn? Does anyone?

Tonight is one of the best examples imaginable. We have to be confident, we have to believe. After all, we have a two-goal lead from the away leg, so why shouldn’t we? We rarely give away goals, apart from the obvious recent aberration, and if you don’t believe in yourself, then you will not win anyway. So why worry?

To quote an old friend, the phrase you must never proclaim at the top of your voice in these circumstances, or even think privately, is ‘nothing can go wrong now.’ Because of course everything can go wrong if you get too sloppy, too comfortable, too arrogant or sometimes just too unlucky. Remember Barcelona in 2009? Bad luck can strike at any time.

Thomas Tuchel will be perfectly aware of the circumstances and will be working on every player and every attitude. Personally, I don’t think there will be any problems with the players’ outlook, but I also know that you can’t be too careful. This is an amazing opportunity to get into the last four of the Champions League, something many big clubs like Arsenal and Spurs can only dream of at the moment. This could and should be a special and historic night for the club, but only if the collective attitude is spot on.

I do not have the tiniest bit of superstition in my bones, but even I will not consider talking about what happens if we go through. That discussion about those teams in the other match only happens after 10pm if we get the right result.

As I write, we are still basking in the glow of a classic performance away at Selhurst Park. Crystal Palace were blown apart by some extraordinary and often viciously precise waves of attacking play.

Most of the discussion has been about Kai Havertz and his display, with Christian Pulisic just behind him in the media coverage, and rightly so. It did seem like another brilliant partnership had been put together and it suddenly flourished on the day. A few weeks ago, I was raving about how good Calum Hudson-Odoi and Mason Mount worked together, and that understanding has not disappeared, but other partnerships have also been built and improved throughout the team.

There is a decent argument that right now, Jorginho and Mateo Kovacic are having some of their best games together as a partnership. Well, they must be doing well, because not only have they frequently kept N’Golo Kante out of the starting XI, but they have also kept a ‘top form’ fully fit N’Golo Kante out of the team. Now that is impressive!

The partnerships are gelling all over the team, but if Havertz and Pulisic can build on this one, it could be very special indeed. Both have the ability to become world class players individually, so just think what they can do as a duo.

I have always felt that it is not a case of ‘if’, but simply ‘when’, with Kai. With any luck the answer to the next question - how soon will it happen regularly? - will be answered by quoting a chap called Morrissey. ‘How soon is now?’

If both are ready to take the next step right now, then anything is possible over the next few seasons, and in the short term, tonight should be safe enough. A single goal from either would mean Porto have to score three to win, a very big ask indeed.

I just hope that Christian can stay free from injury for a decent period. There have been a few times already in his Chelsea career when you thought, ‘This is it, now he is going to go on a goals and assists splurge.’ Then there is a niggling injury, and it has been back to a slow build-up again. Against Palace he finished with the speed, precision and heartlessness of a cobra in a foul mood. This is exactly what you want from an attacker.

It is impossible to tell which front three the manager will start with tonight, but he must have been excited with what he saw from those two at the weekend. He will definitely be tempted to give them another run together very soon, if not this evening.

Sometimes there are little moments for players that tell you something has clicked into place. The finishes from Pulisic against Palace were obviously just such moments. With Kai, it was something else for me. Tt was not his goal, nor indeed the audacious back-flipped kick over the defender that nearly gave him a second.

The moment for me was at the end of his goal celebration. Kai is ridiculously reserved on the field in comparison to most strikers; he doesn’t get too openly excited by scoring goals. I get where he is coming from. It’s an ‘I’m only doing my job, it’s what I do’ kind of vibe going on. On this occasion, however, it was slightly different.

He elegantly picks the ball up (in fact elegant is the perfect way to describe his general play when he is in this form), makes the space gracefully and curls the ball into the far corner in a cultured, almost refined laid-back style. He then jogs away with a calm nonchalance before he finally can’t hold back anymore. The little jump may be partially restrained, and the fist bump half hidden, but the pressure release and joy is there for us all to see.

If he thinks back to that moment and embraces it, understands how he felt, and what it meant to him, then the joy in his game will flow even more. I certainly hope and expect to see that celebration - and maybe an even more unrestrained one - many more times from Kai Havertz in a Chelsea strip.