Bayern Munich's troubling start to Thomas Tuchel's reign continued as the champions slumped to a 3-1 defeat to Mainz at Mewa Arena on Saturday.

Die Roten looked to be on track to bounce back from their midweek Champions League elimination by Manchester City, but they were sensationally pegged back by Tuchel's former team as Bayern's winless streak stretched to four matches.

Sadio Mane gave Bayern a deserved first-half lead from close range, but Mainz showed signs of promise and built on such encouragement after the break when Ludovic Ajorque wiped out the lead.

Leandro Barreiro put Mainz in front 17 minutes from time and Aaron Martin's fine strike soon after finished Bayern off, gifting Borussia Dortmund the opportunity to go top later in the day.

Bayern threatened an early breakthrough when Mane coolly clipped over Mainz goalkeeper Robin Zentner in the 15th minute, only for his goal to be disallowed for offside.

But the visitors did take the lead just before the half-hour mark, Joao Cancelo's left-wing cross leaving Mane with the simplest of headed finishes at the back post.

The hosts were good value for their leveller after the hour, however.

Yann Sommer could only parry Lee Jae-sung's effort and Ajorque was on hand to nod in from a few yards out.

That was merely the start for Mainz.

Eight minutes later, Karim Onisiwo fended off Josip Stanisic when meeting Ajorque's flick-on and subsequently teed up Barreiro, who confidently slotted home.

Martin then put the game beyond Bayern, smashing left-footed into the bottom-right corner from the edge of the box with 11 minutes remaining.
 

What does it mean? Student becomes the master

Mainz boss Bo Svensson played 100 times for Mainz during Tuchel's time in charge and came into this game as the only coach in the club's history with a better points-per-game (in the Bundesliga) record (1.48) than the former Chelsea manager.

He spectacularly got one over his old boss here, making him the first coach to win all of his first three home league games against Bayern since Volker Finke with Freiburg between 1993 and 1996.

The result continued Mainz's brilliant recent run and set a new club record of 10 league games unbeaten, while Bayern's rocky spell rumbles on – Dortmund can take full advantage at home to Eintracht Frankfurt.

Barreiro makes his mark

Mainz's win was arguably as much to do with their willingness to scrap and fight as it was any kind of creative ingenuity, and Barreiro came to sum up their performance.

He was a tireless presence in midfield, his 18 duels contested bettered by only one player on the pitch, and he attempted seven tackles, four more than anyone else. There was also the small matter of his goal, which made it 2-1 and sent Mainz on their way to a tremendous victory.

Sommer under the microscope again

Another week, another questionable performance from Sommer. It was his flimsy save from Lee's strike that gifted Ajorque the equaliser, which had a galvanising effect on Mainz.

It might seem somewhat repetitive for critics to keep pointing to Sommer, but Bayern's collapse here went back to his error.

What's next?

Bayern will hope to finally end their winless run at home to struggling Hertha Berlin next Sunday. Mainz go to European rivals Wolfsburg the same day.