Japan winger Shinji Kagawa scored the late winner in Borussia Dortmund's 2-1 win over Mainz on Saturday to put the defending champions seven points clear at the top of the German league.
This was Dortmund's eighth consecutive win to set a new club record.
"After a brief period when we didn't play so well, we are now as good as we were last year," insisted Dortmund defender Mats Hummels.
Mainz striker Mohamed Zidan, who only left Dortmund in January, scored in his fifth consecutive game for his new club when he found the net from a tight angle on 74 minutes to beat Dortmund goalkeeper Roman Weidenfeller.
"When they play like this, they are almost impossible to stop," said Zidan, who admitted only celebrating quietly against his former club out of respect.
Kagawa restored the hosts' lead when he found space in the Mainz defence just three minutes later.
With 55 points, Dortmund are well clear of Bayern, who are on 48, as Munich's poor form continues.
Bayern suffered their sixth defeat in the league at Leverkusen while they have won only four of their last nine games this year.
They face a make-or-break few weeks and have to overturn a 1-0 deficit from the first leg of their last-16 tie against FC Basel on March 13 in the Champions League before a German Cup semi-final at Gladbach on March 21.
Leverkusen took the lead on 79 minutes when ex-Germany striker Stefan Kiessling tapped the ball home from two yards after Swiss forward Eren Derdiyok's overhead kick took Bayern goalkeeper Manuel Neuer out of the game.
"My team demonstrated a great attitude, they really wanted to win and we earned it because of the second-half performance," said Leverkusen coach Robin Dutt, whose team face Barcelona in the Champions League on Wednesday.
"We played really well in the first half and created lots of good scoring chances, which we didn't take," said Bayern coach Jupp Heynckes.
"That's the crux of our problems at the moment. It was the same for us in Basel.
"It can also happen that the chances go in against you when you don't take your own, which is a real pity for my team.
"The result is very disappointing for us all, but we only have ourselves to blame."
The Bavarians are bidding to avoid a repeat of last season, when they finished in the rare position of having won no silverware.
Hertha Berlin finally broke their seven-match losing streak with a 1-0 win over Werder Bremen as coach Otto Rehhagel, who steered Greece to the Euro 2004 title, enjoyed his first win in only his second game in charge.
Source: AFP Global Edition