The Royal Blues have risen from sixth before the Bundesliga winter break to fourth, establishing a four-point cushion over fifth-placed Wolfsburg in the process, but Keller says they must not rest on their laurels.
"The team's in good shape and are very focused in training, and they're really enthusiastic," Keller said at a press conference. "The positive atmosphere is helping the lads too, but we've not achieved anything yet."
One potential distraction on Friday night could be Schalke's Champions League clash with Real Madrid next week, although Keller insists he is not yet thinking of that game.
"Our total concentration belongs on this game alone," he said. "I'm not going to allow anybody to talk about Real Madrid yet.
"All that matters now is getting a positive result against Mainz. They have a good team with strong forwards like Shinji Okazaki and Nicolai Muller and we're going to have to be incredibly vigilant."
Okazaki and Muller are not Cristiano Ronaldo and Gareth Bale, though, and Mainz coach Thomas Tuchel is aware of that.
"We know what role we play in these games," Tuchel said. "We nevertheless believe we can play a great game, although we know what we're going to need to win.
"At least we don't travel there with any worries, meaning we can focus on our performance and that's a great position to be in."
Tuchel could have Christoph Moritz available, despite him picking up a head wound last weekend, while Park Joo-ho is hoping to have recovered from a thigh strain.
Julian Draxler could return for Schalke, although he is likely to start on the bench after missing almost two months due to injury.