Jurgen Klopp's men disappointed once again as they fell victims to last season's surprise package Hannover in an exciting game with a dramatic finish.
After a quiet first half the game exploded into life in the second as Shinji Kagawa gave BVB the lead, only to see his strike canceled out by Karim Haggui, and rendered a mere consolation just before the final whistle by a wonder goal by Didier Ya Konan.
Dortmund started the game well, pressuring the opposition high up the pitch in search of an opening in Hannover's defense.
Robert Lewandowski was the first to threaten the host defense as in the 16th minute he was released by Ilkay Gundogan with a sublime pass on the edge of the penalty area, but failed to strike the ball with his left foot after controlling with his right.
Ten minutes later Ivan Perisic nearly replicated his goal in midweek against Arsenal as he struck a fierce volley from the left wing, but Zieler did well to parry the effort.
Soon after the champion had Neven Subotic to thank as the defender made a last-ditch stop on Ya Konan after the Ivorian rounded goalkeeper Roman Weidenfeller.
Hannover's Zieler once again proved to be an impenetrable wall for BVB as the goalkeeper rushed to claim a speculative chipped pass by Kuba, which was dangerous floating over the Dortmund defense.
The 96ers managed to pick themselves up after the break and created a few decent chances for themselves. Jan Schlaudraff and Ya Konan were slowly beginning to click upfront, with the Ivorian twice denied by Weidenfeller in the 55th minute.
BVB started to recover and managed to hit Hannover on the break in the 63rd minute through Kagawa, who superbly dribbled past Emanuel Pogatetz before sending a sublime chipped finish into the bottom left corner.
Dortmund continued to press and was wondering how it failed to take the lead 10 minutes later when substitute Kevin Grosskreutz inexplicably missed an open goal from five yards out.
Hannover pressed in the remaining minutes for an equalizer, and after Mats Hummel had gone off with an injury for the visiting team, Haggui proved to be the unlikely hero as he managed to equalize with a header from a corner on the right.
The comeback was completed a minute later when Ya Konan scored with a majestic volley into the left corner from outside the box to seal three important points for the home side.
Dortmund had a numerical advantage in the final minutes after Artur Sobiech was sent off, but it was too little, too late and the game finished 2-1, leaving the champion side's title aspirations hanging by a thread even at this early stage of the season.