Borussia Moenchengladbach have consigned Dortmund to their fifth straight away defeat, beating them 4-2 at home on Friday.
Gladbach controlled the game from start to finish and had several chances to add to their lead in a dominant second half.
Dortmund entered the match smarting after a disappointing 2-0 loss away at Wolfsburg midweek, with manager Edin Terzic pledging to "leave it all on the pitch", but it was Gladbach who started furiously.
Gladbach midfielder Jonas Hofmann, the only member of his side to be named in Hansi Flick's Germany side midweek, got on the board after just four minutes to put Dortmund on the back foot.
Dortmund hit back 15 minutes later when Julian Brandt superbly controlled a pass from English teenager Jude Bellingham on his thigh, before sliding the ball past Gladbach keeper Jan Olschowsky on the spin.
Gladbach reclaimed their lead shortly after through rumoured Dortmund transfer target Ramy Bensebaini, before French striker Marcus Thuram added another four minutes later to put Gladbach 3-1 up.
Qatar-bound Germany defender looked to have ignited a possible Dortmund comeback late in the first half when he scored his side's second, but a debut goal from French midfielder Kouadio Kane in the first minute of the second half saw his side reclaim their two-goal lead.
Hofmann almost had a second in the 69th minute when put through on the counter by Thuram, but the goal was ruled out for a foul in the build-up on Hummels.
After the match, Hofmann told DAZN his side would celebrate the "ultra nice" Friday victory.
"To win like this - and on a Friday - is ultra nice. We can now sit back and watch (the rest of) the Bundesliga."
Dortmund midfielder Brandt said his team "conceded too many easy goals".
"We made Gladbach incredibly strong."
"That's difficult to carry - it's a shitty feeling."
Terzic said his side would need to live with the "disappointing week" through the winter.
"(Now) we need to spend the winter in place six on the table. We don’t know exactly what a win would have meant, but we would have been in place three or place four – and we would have been in the middle of it (the championship race).
"Now, next year, in January we are not starting at zero - we are starting with a minus."
Gladbach have now won their last three games against Dortmund at home.
Bayern Munich have a chance to go nine points clear of Dortmund when they travel to face rock-bottom Schalke in Saturday's late game.