Two first-half goals led promoted Mainz 05 to a surprise 2-1 victory over Bayern Munich in the Bundesliga on Saturday.
Bayern, again without injured playmaker Franck Ribery, are still looking for their first win under new coach Louis van Gaal and are down in 12th place with two points from three matches.
Bayer Leverkusen went top on goal difference after a 5-0 drubbing of promoted Freiburg. Schalke 04 are second following a 0-0 draw at Hoffenheim on Friday while champions VfL Wolfsburg entertain Hamburg SV on Sunday.
Hosts Mainz got off to a good start and striker Aristide Bance went close in the 20th minute.
Five minutes later their aggressive opening paid off when Bayern keeper Michael Rensing was beaten by Andreas Ivanschitz's left-foot shot from the edge of the box.
Burkina Faso international Bance made it 2-0 in the 38th, heading in a Florian Heller cross from five metres.
Van Gaal had already made two substitutions by the 39th minute, including bringing on Ivica Olic as a third striker, in a desperate effort to turn things around.
Bayern pulled a goal back two minutes after the restart when Philipp Lahm crossed and Nikolce Noveski put through his own net.
The visitors pressed for an equaliser but could not avoid their first league defeat of the season.
Visiting Bayer Leverkusen hit four second-half goals to rout Freiburg.
German international Stefan Kiessling put them in front before Swiss duo Tranquillo Barnetta and Eren Derdiyok grabbed two goals apiece.
VfB Stuttgart drew 1-1 at Borussia Dortmund, Georg Niedermeier levelling with a header after Nelson Valdez had fired the home team into the lead.