Leaders Bayern Munich crashed to a 3-1 defeat at Borussia Moenchengladbach Friday as the Bundesliga resumed after the winter break.
Patrick Herrmann scored twice to sink Bayern and lift his side into second in the standings, a single point behind after 18 games.
Third-placed reigning champions Borussia Dortmund can draw level on points with Borussia Moenchengladbach with victory at Hamburg Sunday, likewise Schalke 04 in fourth, who are at home to Stuttgart Saturday.
Emerging star Marco Reus, who will be joining Dortmund next season, set his current side on their way to their stunning victory with the opening goal after 11 minutes.
The 22-year-old seized on a mistake by Bayern keeper Manuel Neuer to curl the ball home, his 11th goal in 16 Bundesliga games.
Herrmann opened his account near halftime after receiving a fine through pass from ex-Germany striker Mike Hanke.
Bayern were clearly missing the suspended Franck Ribery and the home side went further ahead with 20 minutes left as the excellent Reus played in Herrmann for his neatly-taken second.
Bastian Schweinsteiger, returning for Bayern after breaking his collarbone in November, scored a late consolation for the well-beaten leaders.
But Borussia held on to complete the double over the Bavarian giants and blow the German title race wide open.