Leaders Bayer Leverkusen defeated 10-man Union Berlin 1-0 on Saturday and could claim their first-ever Bundesliga title next weekend at home against Werder Bremen.
Florian Wirtz scored the winner shortly before the break to give Leverkusen a 16-point advantage over second-placed Bayern Munich, who were up 2-0 but conceded a shocking 3-2 defeat to promoted Heindeinhem.
Leverkusen, who are unbeaten in all their 41 season matches and a record-equalling 28 in the Bundesliga in 2023-24, can end the title race with a home win against Bremen next Sunday, no matter the result of Bayern v Cologne on Saturday.
If Bayern lose to Cologne, Leverkusen will be crowned champions before their own game. The Bavarians will be without coach Thomas Tuchel, who picked up a yellow card during the game at Heidenheim.
"We're of course looking forward to the Bremen game, but before we still have the Europa League. We're not focusing only on the German Cup and the Bundesliga," sporting director Simon Rolfes said.
Leverkusen are in the quarter-finals of the Europa League against West Ham and play the first leg on Wednesday. They have also secured a spot in the Cup final.
Leverkusen have been in the Bundesliga since 1979, and won the UEFA Cup in 1998 and German Cup in 1993. But they have not lifted a trophy since then, with several near misses including three lost Cup finals and five Bundesliga runner-up finishes - which earned them the nickname Vizekusen (Vicekusen) in Germany and Neverkusen abroad.
Their last defeat was on May 27, 2023, in the Bundesliga finale, 3-0 at Bochum. They finished sixth in a remarkable rise from second last when coach Xabi Alonso replaced Gerardo Seoane in early October 2022, before embarking on their stunning run in the new campaign.
Elsewhere, RB Leipzig thrashed Freiburg 4-1, Mainz defeated Darmstadt 4-0 and Cologne scored two goals in stoppage time to claim a 2-1 comeback win against Bochum. Borussia Dortmund host VfB Stuttgart later.
Alejandro Grimaldo in the 10th minute and Borja Iglesias in the 15th were both denied by Union goalkeeper Frederik Rönnow as Leverkusen were yet to have a clear goal chance.
Grimaldo tired again in the 29th, but his shot rolled centimetres past the post.
In the third minute of added time, Union defender Robin Gosens was sent off after a second yellow card for his foul on Nathan Tella.
After the resulting free-kick, Piero Hincapie's shot was nudged onto the post by Christopher Trimmel's arm, but Odilon Kossounou found the net. The goal was ruled out for offside, but Leverkusen were still awarded a penalty due to Trimmel's offence and Wirtz hit the net from the spot to give Leverkusen the lead.
Rönnow palmed Amine Adli's strike wide in the 59th, but Leverkusen didn't do much more after the break, while Union didn't pose a threat.
The league leaders celebrated the return from injury of top scorer Victor Boniface, who played his first league game in 108 days.
In Heidenheim, Thomas Müller made his 700th game for Bayern and only former goalkeeper Sepp Maier has more appearances for the club with 709.
The Bavarians looked set to give Müller a win in his special game when they finally broke the deadlock in the 38th. Serge Gnabry lifted the ball for the onrushing Harry Kane to break Heidenheim's resistance.
Gnabry got himself a goal before the break, completing a cross from Alphonso Davies and increasing Bayern's lead.
But the collapse of the German record champions started early in the second half as Heidenheim scored two goals in less than two minutes.
Bayern were caught out by a long ball from goalkeeper Kevin Müller and substitute Kevin Sessa went past Dayot Upamecano and through on goal for a finish in the 50th.
The guests were again caught off guard by another long ball and this time Tim Kleindienst was there to hit the net in the 51st.
Keeper Müller denied Kane in the 56th and the English striker lashed across the goal from just inside the area in the 69th, before Heindenheim started getting dangerous again.
Sessa's header in the 72nd was stopped by Ulreich, while Jan-Niklas Beste's free-kick sailed over the crossbar in the 75th, but Heidenheim managed to stun Bayern in the 79th as Kleindienst completed his brace.
