Leaders Schalke 04 surrendered their unbeaten Bundesliga record when they lost 1-0 at promoted Cologne on Friday.
The visitors got away with a series of defensive lapses until Youssef Mohamad headed in for Cologne just before halftime after a long throw from Pierre Wome bamboozled the defence.
Schalke remain top with 11 points from six matches followed by 1899 Hoffenheim, VfB Stuttgart and Hamburg SV on 10 points from five.
Hoffenheim visit Werder Bremen on Saturday while Stuttgart travel to Borussia Dortmund and Hamburg host Borussia Moenchengladbach.
Champions Bayern Munich, eighth on eight points after losing 5-2 at home to Werder Bremen last weekend, will attempt to make up for one of their worst defensive displays in years when they visit Hanover 96 on Saturday.
Schalke beat Hanover 2-0 in the German Cup on Tuesday but a third match in seven days appeared too much for them.
Milivoje Novakovic twice got clear on goal in the first half, with goalkeeper Ralf Faehrmann thwarting him each time.
Nemanja Vucicevic also went close for Cologne before Wome's brilliant long throw led to the opening goal.
Cologne missed chances to kill the game off on the break and they were nearly punished when Schalke substitute Halil Altintop headed against a post at the end.
