Barca came from 2-0 down to draw 3-3 with Inter at Montjuic.
Flick said afterwards: "I don't think we started well; we conceded two goals, but we fought back. We had ups and downs in the first half. The second half was spectacular for the players and the spectators. It's the first meeting between the two. Now we have to win in Milan. It's a final before the final.
"We're talking about a Champions League semi-final, with great teams. They have experience. They showed it. They played very well from set pieces.
"I think everyone has seen the team's performance. With the ball, they've been incredible. They've been fantastic. We didn't start well, conceding two goals. We conceded two goals from set pieces. They have height. We have to improve. We're in the same situation as in the Cup against Atlético Madrid.
"I didn't say anything to them today. I think it's good for them to sleep and we'll talk tomorrow. I appreciate the style, we went for the goal... we came back from being two goals behind. Lamine was very important, he did a lot of positive things... and he scored the goal."
Yamal was doubt before kickoff
Indeed, on goalscorer Lamine Yamal, Flick revealed: "He felt a discomfort in the warm-up, but we saw him and gave him the green light. He's a fantastic player, everyone knows that. In the big games, you see the quality of this player. It's good to have him. He's special. He's a genius. When you see the details he does, it's incredible.
"It shows that he's an important player. He's shown how good he is; that's what we need. He's there in the big games. I think he's enjoying this situation. I'm very happy that this talent, who comes along every 50 years, is playing for Barcelona."
Flick added, "(Proud?) Of course. When you play every three days, it's important to train, but there are injuries... the team will give its best. Everyone has seen what I have, that the performance has been fantastic.
"We want to dominate, but things happen because the opponent also plays and has their automatic behaviors. They're fantastic from set pieces, they're a very good team."