The Bavarians suffered on their own turf as they lost for just the second time in the current league campaign, giving BVB a chance to pull within two points
Bayern Munich coach Pep Guardiola said his side's shock 2-1 loss to Mainz at the Allianz Arena proves that nothing can be taken for granted in the Bundesliga.
Not even by Bayern, who are aiming for a fourth straight league crown.
The loss, coupled with Borussia Dortmund's 2-0 win away to Darmstadt, has seen their lead at the top of the table reduced to five points.
The top two meet on Saturday in a match where Dortmund could cut the margin to just two points and Guardiola said that history was against his side.
"We clearly saw tonight that nothing is easy," Guardiola said.
"No team has won the Bundesliga four times in a row and Dortmund's team is totally different from the last three years.
"We must now think of the next game."
Speaking of the performance against Mainz, Guardiola was less than impressed by what his team produced, but was not overly disappointed.
"Our performance was not good enough, but I'm not sad," he said.
"I congratulate [Mainz coach] Martin [Schmidt] on the victory."
- Goal