Schalke 04 lie only two points behind Bundesliga leader Bayern Munich after defeating Arminia Bielefeld 2-1 on Sunday.
Schalke's scorers, Ailton (left) and Levan Kobiashvili
Under manager Ralf Rangnick Schalke have collected an impressive 24 points from a possible 27 and are now Bayern's closest title rivals.
Schalke fell behind to a Rüdiger Kauf strike in the 20th minute but hit back before the break with goals from Levan Kobiashvili and Brazilian striker Ailton, Reuters reported.
Goalkeeper Frank Rost secured victory for below-par Schalke when he saved Detlev Dammeier's penalty in the 55th minute.
Schalke's success had some Bayern officials a little nervous.
"Perhaps Schalke will get near us on the pitch but in financial terms they are 20 years behind," said Bayern general manager Uli Hoeness. "We have as much credit as Schalke have debt."
But historically, the top team at Christmas usually goes on to win the German crown in May, as was the case with Werder Bremen last season. There are two match days left before the five-week winter break.
VfB Stuttgart meanwhile remains in third place despite Sunday's goalless draw against Hanover 96.
On Saturday, Bayern, who went top of the league for the first time in 18 months last weekend, saw their domestic purple patch continue only days after their 5-1 Champions League win over Maccabi Tel Aviv. VfL Wolfsburg, who went down 3-2 to Hertha Berlin, dropped to forth place.
Bayern manager Felix Magath made four changes to the team that crushed Tel Aviv in midweek, but that did not help Mainz. After just 14 minutes on-form Bayern striker Claudio Pizarro picked up a pass from Brazilian Ze Roberto and rolled the ball into the net.
The goal visibly lifted Bayern and Mehmet Scholl, the star of the show against Tel Aviv, doubled the lead in the 35th minute. Dutch international Roy Makaay fed a sublime pass into the path of Scholl who defied his 35 years with a great turn of speed to slide home from an acute angle.
Mainz were camped in their own half and Makaay scored in the final seconds of the first half to make it 3-0. Bayern eased off in the second half and three goals came in the latter stages. Showing a never-say-die spirit, the two goals from the newly promoted Mainz came in the final six minutes.
Bremen beat Dortmund
In this campaign Bremen are left clinging to the coat tails of Bayern after a 2-0 win over Borussia Dortmund. The French connection of Johan Micoud (32) and Valerien Ismael (57) helped sink Dortmund and Bremen are now eight points adrift of Bayern.
Wolfsburg had the best home record in the league before facing Berlin but the capital outfit showed no respect with a 3-2 triumph at the Volkswagen Arena. The Wolves came from two goals down to level at 2-2 but the day belonged to Brazilian Marcelinho who scored a hat-trick (16, 47, 74).
After a creditable 1-1 draw at Real Madrid in midweek Bayer Leverkusen registered a long overdue first away win of the campaign.
Bayer ran out 2-0 winners over Hansa Rostock courtesy of goals from Bulgarian international Dimitar Berbatov (36) and Ukraine ace Andrei Voronin (63). Basement boys Hansa have now lost all eight of their home matches this season and look destined to be watching the Bundesliga on television next term.