Atletico Madrid managed to win when they hosted Borussia Dortmund in this first leg, 2-1 was the final score of the match. Atletico Madrid can take advantage of their status as hosts, because with this win they only need to achieve a draw in the second leg.
However, it is not easy for them to do that, because winning by 1 goal does not make them feel very comfortable. Maybe they are more relaxed, but considering that the second leg match will be held at Borussia Dortmund headquarters, it makes them have to work even harder.