Leverkusen on cusp of first Bundesliga title after Bayern loss
Bayern were cruising at half-time thanks to strikes from Harry Kane and Serge Gnabry but conceded three times in the second half, losing their second match in a row.
Florian Wirtz converted a penalty deep into first-half stoppage time as Leverkusen were made to work hard in the German capital against 10-man Union, extending their unbeaten run this season to 41 games.
Xabi Alonso's side need just three points from their remaining six matches to clinch the club's first league title.
Bayern, on a streak of 11 straight Bundesliga titles, could relinquish their crown next week in between the first and second legs of their Champions League quarter-final against Arsenal.
Union did their best to hold on despite Leverkusen's relentless pressure but gave way just before half-time, Germany defender Robin Gosens picking up a second yellow for a desperation tackle on Nathan Tella near the penalty box.
After some chaotic scenes from the ensuing free-kick the ball connected with the arm of Union captain Christopher Trimmel, giving Leverkusen a penalty after a long VAR review.
Wirtz calmly dispatched the penalty, his third goal in his past two games.
Late in the second half word filtered through to the visiting fans that Bayern were trailing at Heidenheim, sending them into raptures.
"Of course the situation is super but we don't want to celebrate yet," said Alonso.
"We can maybe celebrate something next Sunday. Before that there's a Europa League game (at home against West Ham).
"Nothing is settled yet and we need to wait a little."
Bayern 'drastically punished'
Bayern looked in fine form early on, despite missing several injured first-teamers including captain Manuel Neuer, Leroy Sane and Kingsley Coman, with coach Thomas Tuchel taking a cautious approach with Tuesday's Champions League trip to Arsenal in mind.
The ever-reliable Kane, yet to miss a league match this campaign, opened the scoring, scrambling in a Gnabry assist after 38 minutes.
Gnabry seemingly had Bayern on track for a simple victory when he scored just before the break, but the hosts turned the match around early in the second half.
Kevin Sessa and Tim Kleindienst both scored inside the space of 90 seconds to level the scores and stun the visitors.
Kleindienst struck again with 11 minutes remaining to snatch victory in their first home match against Bayern, a goal celebrated 460 kilometres away in Leverkusen.
"We were drastically punished for not keeping our energy," Tuchel said.
"The manner in which we lost today was too easy. Congratulations to Heidenheim. They turned the game on its head."
In Saturday's late game, Stuttgart won 1-0 at Borussia Dortmund to draw level with second-placed Bayern on 60 points.
Serhou Guirassy scored the only goal of the game, blasting in on the counter for his 24th league goal of the season.
Dortmund defender Nico Schlotterbeck had a golden chance to equalise late but somehow blazed over with the goal beckoning.
Schlotterbeck told Sky the setback would not hurt Dortmund ahead of Wednesday's Champions League quarter-final first leg against Atletico Madrid.
"We've got a nice game waiting for us in Madrid on Wednesday," he said.
"When we play like we did today, I'm positive we will win."
Dortmund's loss means they drop to fifth, pushed out of the top four by RB Leipzig who won 4-1 at Freiburg earlier on Saturday thanks to a brace from Belgium forward Lois Openda.
Amadou Haidara opened the scoring just one minute in and Openda added two more before half-time either side of a missed penalty from Freiburg's Lucas Hoeler.
Openda, who arrived from Lens last summer, now has 21 goals for the league campaign.
Mainz won 4-0 at home over last-placed Darmstadt to keep their hopes of avoiding relegation alive.
They have lost just one of their past five and sit in the relegation play-off spot, five points from safety.
Cologne also boosted their bid to avoid relegation, scoring twice in added time to win 2-1 at home against Bochum.
