Cologne stunned in-form Eintracht Frankfurt 3-0 with two goals from Tunisia international Ellyes Skhiri on Sunday to snap their opponents' unbeaten run in the league since October.
Frankfurt, who host Napoli in the Champions League round of 16 on Feb. 21, are sixth in the standings on 35 points, eight behind leaders Bayern Munich.
Cologne, in 11th place, had played two consecutive goalless draws going into the game but Timo Huebers ended their drought with a glancing header in the 49th minute.
They came close to scoring again six minutes later but Steffen Tigges was denied at the last by Evan Ndicka's well-timed tackle in the box.
With Frankfurt desperate for an equaliser, Cologne struck in the 71st when Skhiri nodded in on the rebound after a lightning-quick counter-attack following an Eintracht corner.
The 27-year-old midfielder rifled in his second four minutes from time, his fifth league goal of the season.
