Arminia Bielefeld continue to be Bayer Leverkusen's nemesis as the visitors relinquished their lead in the Bundesliga following another defeat at the Schuco Arena.
Artur Wichniarek fired the hosts in front in the 53rd minute and Daniel Halfar doubled their lead in the 69th minute.
Patrick Helmes pulled a goal back with 12 minutes left, but any chance Leverkusen had of getting something from the game disappeared when Sascha Dum was sent off three minutes from time.
Leverkusen, who have not won in Bielefeld since November 1999, had their six-game unbeaten streak broken and remain on 28 points - three behind new leaders Hoffenheim.
Bielefeld had not won in eight games and scored just five goals in that stretch, but their second win of the season moves them out of the relegation zone with 12 points - good for 14th place.
The hosts nearly pulled ahead after just six minutes but Leverkusen keeper Rene Adler batted Thorben Marx's blast from 20 yards over his crossbar.
Bielefeld stood very well defensively and did not allow Leverkusen to create many chances in the early stages, but goalkeeper Dennis Eilhoff had to make a fine stop from Helmes in the 17th minute and five minutes later a Leverkusen free-kick was headed just over the bar by a member of the Arminia wall.
The game was back and forth the rest of the first half but the deadlock was not broken until Wichniarek struck eight minutes after the break.
The Polish striker was sent by Halfar down the right wing and then beat Adler from a tight angle with the ball going in off the left post.
Arminia doubled their advantage in the 69th minute as Adler attempted to punch a cross clear but the ball went right to Halfar, who pounced home with a left-footer.
Leverkusen remained poised to make a comeback and cut the deficit in half in the 78th minute when Renato Augusto sent a long diagonal pass to Helmes, who blasted into the corner.
Eilhoff fended away a fine header by Helmes five minutes from time and then indecisiveness between Simon Rolfes and Manuel Friedrich saw Theofanis Gekas' superb headed pass sail through the box untouched a minute later.
However, all chance of an equalizer evaporated when Leverkusen went a man down with two minutes left as Dum was shown red for a crude tackle in midfield.
