Reigning champions Bayern may have crushed Spartak Moscow 4-0 in the Champions League in midweek but that did not matter to Arminia Bielefeld who pulled off the biggest sensation of the season so far with a 2-1 win.

Danish winger Jonas Kamper scored a brilliant late free-kick to condemn Bayern to their first defeat of the season.

"I do not want to make excuses but the match calendar has certainly not helped," said Bayern coach Felix Magath. "There has been an international break, the domestic cup, the Champions League and back to the league.

"I am sure we will find our rhythm soon and not lose games like this."

Bayern's loss means FC Nuremberg lead the 18-strong Bundesliga for the fourth successive week after a 1-1 draw with VfL Bochum.

Dutch international midfielder Marc van Bommel, a six million-euro signing from Barcelona, had put Bayern ahead on his debut just six minutes in with a scrambled goal.

But Bielefeld rallied and Artur Wichniarek equalised in the 25th minute before Kamper stole the headlines with his free-kick.

"We conceded two goals from set-pieces and that can happen to any team," said Bayern's German international Philipp Lahm.

"We did not allow them many chances but we are guilty of not being purposeful enough going forward."

Fortunately for Bayern, who fell to third in the table, their rivals once again failed to capitalise on their slip with Bremen's miserable run continuing with a 3-2 defeat against VfB Stuttgart.

"We had a comfortable 2-0 lead and then we stopped putting tackles in," said a livid Bremen coach Thomas Schaaf. "Now is the time to stop talking and to start taking action."

Bremen, who fall to seventh, hoped to get back on the winning trail after a German Cup loss to third division Pirmasens and a Champions League defeat to Chelsea.

They lost their star striker Miroslav Klose through injury shortly before the match but stormed into a two-goal lead with Roberto Hilbert heading into his own goal and Egyptian international Mohamed Zidan producing a cool finish.

But Hilbert scored at the right end in the 32nd minute and it sparked a sensational comeback as goals from Pavel Pardo and Luis Gomez secured a 3-2 win for the visitors.

"That should not be allowed to happen," fumed Bremen captain Frank Baumann. "We just invited the opposition to score goals."

Title hopefuls SV Hamburg saw their woeful start to the season continue with a 1-0 loss at Borussia Dortmund.

"We are wobbling but we just need to keep pushing on and try to win our next match," declared Hamburg boss Thomas Doll.

It was three defeats in a row for Hamburg who had lost to Stuttgart Kickers in the German Cup and then Arsenal in the Champions League.

Former German international Christian Worns scored a late winner for Dortmund and Hamburg, third last season, now lie fourth from bottom without a win to their name. Dortmund are the big climbers going from ninth to fourth with the win.