The seven-time Bundesliga champions moved level on points with second-placed Borussia Dortmund thanks to goals from Max Meyer and Jefferson Farfan at the Veltins-Arena on Friday.
Schalke are 10 points clear of Borussia Monchengladbach in third position and in the box seat to qualify for next season's UEFA Champions League with four matches to play.
After a goalless opening 45 minutes, Schalke had to wait until the hour mark to break the deadlock - something that did not please Keller.
"I was not satisfied at all in the first half because we did a lot of things wrong," said Keller, who has guided the club to a seven-match unbeaten streak.
"I told my players exactly that at half-time but after that we improved greatly. A lot of respect for that."
Star striker Klaas-Jan Huntelaar was back in the starting line-up after overcoming injury but he was unable to cap his return with a goal.
Huntelaar stepped up to take a penalty after Julian Draxler was fouled inside the box by Carlos Zambrano on 66 minutes, however Frankfurt goalkeeper Kevin Trapp was on hand to deny the Dutchman.
The 30-year-old has now missed four of his last six penalties.
"The chances of him taking another penalty are diminishing," Keller said.
