Bayern Munich midfielder Joshua Kimmich said his side need to question their mentality after they slumped to a shock 4-2 loss at Bundesliga minnows Bochum ahead of Wednesday's Champions League clash with Red Bull Salzburg.
Bayern conceded four goals in the first half for the first time in a Bundesliga match since 1975 as promoted Bochum rampaged to victory, leaving the perennial champions stunned.
"This was our worst performance of the season. We need to ask ourselves whether this is the mentality which Bayern Munich embodies," a dejected Kimmich told Sky.
"Luckily it's happened in a Bundesliga match this time, but we have a Champions League game on Wednesday," he added.
Bayern had not lost to Bochum since 2004 ahead of this game, and beat them 7-0 when the two teams last met in September.
When Robert Lewandowski bundled in the opener for Bayern on nine minutes, it seemed it would be a normal afternoon for the perennial champions.
But Bochum equalised quickly through Christopher Antwi-Adjei, before taking control in a frenzied 10 minutes before the break.
Juergen Locadia gave the hosts the lead from the penalty spot after a Dayot Upamecano handball, and unsettled Bayern wilted under pressure.
Cristian Gamboa smashed the ball into the top corner from the edge of the area two minutes later, before Holtmann curled in another brilliant long-range effort on the stroke of half-time.
"My first Bundesliga goal, and it comes against Bayern. It's like a dream!" Gamboa told Sky.
Lewandowski pulled a goal back in the second half, but it was too little for Bayern to avoid a fourth league defeat of the season.
After a week in which German football discussed a play-off format to combat Bayern's relentless dominance, recently promoted Bochum delivered a brutal reminder that the champions are not unbeatable.
Yet Bayern remain nine points clear at the top of the table overnight, ahead of Borussia Dortmund's trip to Union Berlin.
In-form Bayer Leverkusen remain third after they eased to a 4-2 win over Stuttgart.
Leverkusen had scored ten goals in their previous two games, but were held at bay by a dogged Stuttgart defence until Moussa Diaby slalomed through the box to score the opener just before half time.
New Portuguese signing Tiago Tomas brought Stuttgart level with his first Bundesliga goal, but Amine Adli immediately restored the lead at a free-kick.
Florian Wirtz extended the lead five minutes from time, threading a crafty low shot through a flurry of legs and into the bottom corner.
Tomas gave Stuttgart fresh hope with an instinctive finish at the other end, before Patrik Schick sealed the win with a tap-in.
Elsewhere, struggling giants Wolfsburg and Borussia Moenchengladbach both pulled away from the relegation zone with timely wins.
Wolfsburg beat former coach Oliver Glasner with a 2-0 win at Eintracht Frankfurt, a Max Kruse penalty and a late goal from Dodi Lukebakio handing them a second successive win.
Manu Kone and Jonas Hofmann gave Gladbach a comfortable lead, before Iago pulled a goal back for fellow strugglers Augsburg.
Ramy Bensebaini restored the two-goal lead before Alfred Finnbogason snatched a late consolation goal.
Bottom club Greuther Fuerth gave themselves hope in the relegation fight with only their third win of the season against Hertha Berlin.
Branimir Hrgota opened the scoring after just 27 seconds and doubled the lead with a second-half penalty before Hertha's Linus Gechter scored his first Bundesliga goal late on.
Freiburg leapfrogged Union Berlin into fifth with a 1-1 draw at home to Mainz, substitute Nils Petersen cancelling out Alexander Hack's first-half opener.
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