The Blues face an almost improbable task of overturning a 3-1 deficit suffered in the first leg encounter at Stamford Bridge last week.
A Karim Benzema hattrick put the current Champions League holders on the brink of elimination.
Chelsea dispatched Southampton 6-0 in the Premier League over the weekend and will need such ruthlessness in front of goal to have any chance of advancing to the last four.
But Tuchel knows the trip to the Santiago Bernabeu will be a difficult one, although he has asked his charges to put up a better fight than they did last week.
He said: "It's one of the biggest challenges to perform as an away team at the Bernabeu. If you have to win by at least two goals, it is very difficult, almost impossible. We need to overperform.
He added: "We never manage our input and effort by chances we have for a result, we never did and will not start tomorrow by giving less because it's very unlikely we will make it.
"It's unlikely, but we will try, we will play to our full limit. It's a big night and a big match and we wll try to be in better shape and in a better condition than in the first leg."
"We need nothing else than a fantastic script," he said. "The task is incredibly high after the first leg. You're always allowed to dream and sometimes it is important to imagine and dream about things, but it doesn't change the fact we need to be ready to deliver.
"We will try hard, it's worth trying. Hopefully we can fulfil our dream, but realistically we have to invest a lot."
Chelsea will be without Romelu Lukaku due to an Achilles injury but captain Cesar Azpilicueta is back after recovering from Coronavirus.
