The sides meet in the first leg of their UEFA Champions League last 16 tie at the Veltins-Arena, with La Liga leaders Real heavily fancied against their German opponents.
But Schalke coach Keller insists that the result is not a foregone conclusion, and is hopeful that his men can cause an upset.
"We'll have to be compact at the back to achieve what might be a minor miracle of getting a result," he said. "Real Madrid are one of the best teams in the world, possibly only behind Bayern Munich.
"Our work has been meticulous but no different to any other game. The players are prepared.
"We're competing with a lot of great clubs. Real Madrid have a great team but we do, too. It's about the collective.
"I saw them against Atletico Madrid and they played very well. But our priority is to play as well as we can and are focused.
"We have to be brave. We'll try to stay compact but also to find the gaps up front. It's about the balance.
"We're in good form in the Bundesliga, in terms of results and performances. Real Madrid is a different animal though.
"It's an extraordinary game for us but the preparation is no different to any other. We're all just as focused.
"We're in a position to put in a great performance, and we'll have to. Real Madrid have an unbelievable team."
Keller also dismissed suggestions that the tie would serve as a showcase for Julian Draxler, who has reportedly caught the eye of some of Europe's top clubs in recent months.
"This isn't a shop window for Julian Draxler," he said. "He was injured for a long while and we'll need him in the game."
Former Real Madrid striker Klaas-Jan Huntelaar added: "It was a great time for me at Real Madrid, but now I'm with Schalke and I'm fully focused on the game against Madrid."
Schalke are fourth in the Bundesliga and have not been beaten in seven league matches.
