Hanover stuck in third after loss to St Pauli
St Pauli's Marius Ebbers headed in the winner off the post in the sixth minute as Hanover, who had recorded the club's best Bundesliga start after six games, paid the price for their passive game.
With key striker Didier Ya Konan missing through injury, frustration boiled over in the 80th minute when Hanover's Karim Haggui was sent off for a foul on Matthias Lehmann at the edge of the area.
Hanover are third on 13 points from seven games, with Dortmund, who host champions Bayern Munich on Sunday, on 15.
Surprise leaders Mainz 05, on 18 points, entertain fourth-placed Hoffenheim on Saturday.