Robert Lewandowski struck a hat-trick, with Lamine Yamal also on Barca's scoresheet. The match ended, however, with Frenkie de Jong being sent off.
Flick said afterwards: "I'm satisfied with the team. I'm very happy. I value what we've done. I think we made mistakes in the first half, but we improved in the second; we defended better. There were good things in the first half too. We played a good game with the ball.
"Football doesn't count for what you do in the past. The second half will give us confidence for the future. We're focused on ourselves. We put good pressure on the opponent. I'm happy with the second half."
With Lewandowski off contract in June, Flick also said: "It's too early to talk about the future. Everyone knows his situation. I've seen a different, more positive Robert. He's back ahead of schedule. It's good for his confidence. And it's important for us too. He scored three goals today. He was also very important last season."
On De Jongm Flick added: "He played a good game today, he controlled the match, and what he did was very important. He's playing at an incredible level. It's not good not having him for the next game. He's an important player, but we have to manage him. We'll see if Raphinha, Pedri, Joan García... return."