Atletico won 2-0 in the first-leg of their quarterfinal thanks to goals from Julian Alvarez and Alexander Sorloth. Pau Cubarsi was sent off - via VAR - for Barca before Alvarez's opener.
Afterwards, Simeone said: "We defended in an organised way so they couldn't advance. We couldn't create as many chances as we would have liked, but then the one we were looking for finally came.
"I was far from Cubarsí's action, but VAR called the referee over. Julián scored an incredible goal. In the second half, they looked more for counter-attacks."
Asked if they're already in the semifinals, Simeone also said: "No. We are facing a very strong opponent, and we are humble enough to say that they are very good. We will play with great enthusiasm, with our fans, and with commitment.
"I prefer to be on the side of the advantage. After the 4-0 (in the Copa del Rey semifinal) we already saw how they responded at home. We imagine a difficult and complex match."
It was a first for Atletico at the Nou Camp since 2006.
Simeone added: "I'm happy for the players' effort. Today we were able to confirm an important match and, with humility, we know it will be a tough match at home."