Atletico will meet Dortmund in the first-leg of their Champions League quarterfinal.
Witsel said: "In the end we think game by game, it is the most important thing. We can't think any further. The first thing is this meeting and then we can think in another way.
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It is a very good team, I played four years there, they were very good for me. They press a lot, they have very fast players, they have players who can make the difference.
We only think about this game. We cannot go beyond this. We think like Cholo. Game by game, we cannot think beyond tomorrow's game.
It is an issue to improve. We know. It's collective and tomorrow it will be better. We need to improve."