When the full-time whistle went at the Santiago Bernabeu on Saturday night, the Atletico Madrid players had huge grins on their faces, and rightly so after beating Real Madrid in their own back yard for the second time in four months.
The manager, Diego Simeone, surely had a grin across his face too. Yet we can't know for sure as he was away up the tunnel almost as soon as the final whistle blew, perhaps to celebrate in private as the Argentine didn't want to break his serious persona.
Yes, Atletico have won their first seven La Liga matches - equalling the La Liga record along with Barcelona, who picked up their 7th win the same day - and they only lost the Super Cup to Barca on away goals, but Simeone knows that they have achieved nothing yet and there can be no public gloating until they do.
Atletico led their Madrid rivals for the majority of last season, only to finish behind them in the end. They now lead Real by five points - a healthy advantage, but one that can be clawed back.
And nobody on this team seems to be getting carried away with themselves. There is no talk of winning the league, nor any of finishing second. Instead the players and manager have acknowledged the positive things written about them in both the Madrid and Barcelona press, but are instead thinking just one game at a time.
That next game is away to Porto and is just as important, if not more so, than the Saturday night success at Real. A slip up here and all the positive talk in the press will come to an end.
You might think that coming back from Porto with anything less than a win could hardly be called a "slip up" and that would be a fair point. Porto haven't lost a home game in the league or in Europe since February 2012.
However, Atletico will never stop being judged and can only be considered a real contender domestically and in Europe if they can consistently beat the big names. That means following up a win at the Bernabeu with one over another Iberian giant.
It will be tough, not only because it's an away trip to Porto, but because Atletico will be without star striker and derby-scoring hero Diego Costa, who serves the last match of a UEFA ban.
David Villa will, therefore, lead the line and he is no stranger to big European nights having scored once in the biggest of them all - a Champions League final.
And, with Belgian keeper Thibaut Courtois recovering from a knock with Gareth Bale, this side certainly has the talent to leave the Dragão with a win, just as they sent Zenit St Petersburg home defeated in the first group game without Costa.
A point would be a good result. A win would keep the optimism around the Vicente Calderon stadium at an all-time high.
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