Promoted club Frankfurt are just four points and two places behind Schalke in the league, and Keller wants to make sure that his side create a gap by beating the teams around them in the league.
"We face a difficult task at Frankfurt," the club's official website quotes Keller as saying. "They've stuttered a bit lately but overall they're having a really good season for a promoted side.
"How close the chasing back get to us is down to us. We've fared better recently against teams in the top half of the table."
Schalke are four points behind third-placed Bayer Leverkusen.