The Spaniards overturned a 1-0 first-leg defeat to Porto to go through on aggregate 2-1, and Demichelis urged his teammates not to get carried away with their chances.
"We will enjoy this because of the effort we put in tonight but even more so because of the [bad] image we gave of ourselves in the first leg," UEFA.com quotes Demichelis as saying.
"On that day, we failed to show what this team is capable of. We reacted well in the second leg and played intelligently.
"Getting to the quarter-finals with Malaga is a dream turned into reality. We must maintain our humility and stay grounded now; the euphoria is for the fans. We will enjoy Friday's draw."
Demichelis has made 31 Champions League and La Liga appearances for Malaga this season.