The Argentine says that his side played well in against Valencia, but says that Unai Emery's men will be capable of overturning the tie at Mestalla
The Rojiblancos will take a commanding lead to Mestalla in a week's time, but Simeone feels their participation in the final of the competition is far from assured.
"The win is not decisive, but it's a result that we all would have taken beforehand," he told AS in the wake of the game.
"We will go to Valencia knowing it be tough, especially in a great atmosphere."
The Argentine went on to praise his team's ability to mix aggressive play with attractive football.
"The team showed their best side, and were aggressive at all times, but the goals were all the result of good play."
Meanwhile, Radamel Falcao, who scored twice in the encounter, was quick to echo his coach's warning against complacency.
"Great effort tonight!!!" the Colombian Tweeted. "We got [sic] at least 90 minutes ahead, we should be very careful to protenct our advantage! C'mon Atleti!"
Atletico Madrid will now turn their attention to domestic matters, with Espanyol visiting Vicente Calderon on Sunday.