Schalke are into the last 16 of the UEFA Champions League but, prior to their European clash, they must negotiate a home league fixture with seventh-placed Mainz.
Keller's side occupy a Champions League qualifying spot for next season as it stands, as they sit in fourth - four points clear of fifth side Wolfsburg.
Four straight wins has Schalke sitting pretty in the German league but Keller warned Mainz's unpredictability could be their undoing.
"Well Mainz is capable of playing different systems during a game and switching between them," he said.
"They are quite aggressive with the ball, and very compact as a team. With (Nicolai) Mueller, they have a very quick player for an offensive game, so is (Shinji) Okazaki.
"They deserve (to be) where they currently are. They do have plenty of quality as a team. That's what makes them dangerous for us."
Keller said the last-16 first leg with Madrid on Wednesday was out of the players' heads, with the league fixture their first priority.
"Our team is focused solely on the match against Mainz on Friday, that's what counts, that's courtesy of the current situation in the table," Keller said.
"The team knows how the situation will be, if we beat them.
"Right now, Madrid is not an issue at all, I have made it clear to my team as well.
"I feel that the guys are sharing my point of view and focusing completely on Mainz.
"On Saturday you can start talk about Madrid but not now."
Schalke will host the first 90 minutes against Madrid, before heading to the Bernabeu on March 18 for the second leg.
