(CNN) -- Bundesliga leaders Schalke surrendered their unbeaten record on Friday when they lost 1-0 at promoted Cologne.
Youssef Mohamad (far right) is congratulated by his team-mates as Cologne shocked Schalke 1-0.
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 confused the Blues' defence.
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 on both occasions.
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.
Schalke remain top with 11 points from six matches followed by Hoffenheim, Stuttgart and Hamburg 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 recent years when they visit Hanover on Saturday.