Third-place Werder Bremen remained level on points with Dortmund after a second-half hat-trick from Claudio Pizarro saw them come from 2-0 down to beat 10-man Cologne 3-2.
Cologne led early through Christian Clemens' third minute strike and it was 2-0 after 45 minutes when Lukas Podolski struck.
Pizarro pulled a goal back in the 49th minute before equalising from the spot on the 54-minute mark when Henrique Sereno was red-carded.
Pizarro then finished off his hat-trick, and won the game, in the 86th minute.
On Friday night, Mainz picked up their first league win since mid-August with a 3-1 victory over Stuttgart at the Mainz Arena.
Not even a red card for Eugen Polanski was able to stop Thomas Tuchel's side from halting a slide which had had lasted nearly three months since their last victory over Freiburg on August 13.
Cacau put Stuttgart in front in the 50th minute, but Anthony Ujah was on target twice and Andreas Ivanschitz scored a penalty as the home side moved three points clear of the drop zone in front of another sell-out crowd.
The visitors' let their frustrations get the better of them after the final whistle and Francisco Maza was shown the red card for confronting the referee as Mainz began celebrating a long overdue win.
