The two teams played out a goalless draw at the Vicente Calderon last night, with the second leg to be held in London next week.
"We expected a tough game and we knew they were going to hold the 0-0," he told AS.
"We tried to play a great game and look for the goal at all times but it wouldn't come. The result could be good for either of us. The game will be defined in London.
"We're 50% favourites to go through. They have a slight advantage playing at home but it's still 50%. We must try to score like we did here because there it would be worth double."
Any win or score draw would see Atletico advance into the final.