Bayern Munich crashed to a 2-1 defeat at Eintracht Frankfurt on Saturday in the Bundesliga as the world champions again showed chinks in their armour before they return to Champions League action next week.
Robert Lewandowski netted his 26th league goal of the season for Bayern, but Frankfurt deserved the win thanks to first-half goals by Japan midfielder Daichi Kamada and Amin Younes.
After their 3-3 league draw at home to strugglers Arminia Bielefeld last Monday, Bayern again laboured ahead of Tuesday's Champions League last 16, first leg tie at Lazio.
"We haven't learned from the Bielefeld game," Bayern captain Manuel Neuer told Sky.
"We have to be aggressive from the very beginning, we have to be wide awake. We only got going in the second half."
Frankfurt's win gives hope to Bayern's league rivals as second-placed RB Leipzig can cut their lead to two points if they win at Hertha Berlin on Sunday.
It was Bayern's third league defeat. Their last, a 3-2 reverse at Borussia Moenchengladbach in January, was followed by a shock German Cup defeat at second-tier Holstein Kiel.
In a sign of Bayern's domestic dominance, it was only the sixth defeat head coach Hansi Flick has suffered since taking charge in November 2019.
Frankfurt capitalised on Bayern's jaded form since they lifted the Club World Cup in Qatar just over a week ago.
Bayern were missing defender Benjamin Pavard plus forward Thomas Mueller, who were both sidelined after testing positive for Covid-19, while Corentin Tolisso is likely to be sidelined for months after injuring himself in training this week.
Frankfurt's top scorer Andre Silva dropped out before kick-off with a back injury.
Just like in Monday's draw against Bielefeld, Bayern were 2-0 down at half-time after Kamada opened the scoring by tucking away a cross after 12 minutes.
Kamada then set up Younes, who fired into the far corner past the diving Manuel Neuer on 31 minutes.
Bayern pulled a goal back just after the break when Lewandowski scored after Leroy Sane toyed with his marker before putting in a superb low cross.
Despite Bayern's second-half onslaught, Frankfurt held on for a fifth straight league win which cements their hold on fourth in the table.
Elsewhere, Borussia Moenchengladbach crashed to a 2-1 home defeat to Mainz before their Champions League last 16 first-leg clash against Manchester City in Budapest on Wednesday.
Karim Onisiwo's opening goal was cancelled out by Lars Stindl's equaliser for Gladbach before Kevin Stoeger scored Mainz's winner four minutes from time.
It ended a week to forget for Gladbach who on Monday announced that head coach Marco Rose will leave at the end of the season to take charge of Dortmund.
"The defeat is extremely annoying, it's really hard to accept," Gladbach midfielder Jonas Hofmann told Sky after their first defeat to Mainz for five years.
Stuttgart, whose head coach Pellegrino Matarazzo on Friday extended his contract until 2024, celebrated with a 1-0 win over Cologne thanks to Sasa Kalajdzic's goal.