DW's Ross Dunbar looks at why Porto came away with a 3-1 win and applauds the Portuguese side's tactics and ideas on the night. Bayern need to overcome a two-goal deficit in the return leg.
The message from the FC Porto fans was clear: "Respect!" said one part of a pre-match choreography that was geared for the 'Road to Berlin.'
And respect was exactly what was missing from Bayern. The Portuguese side was strategically stronger and possessed devastatingly effective players on the counterattack with speed, power and poise.
Led by Jackson Martinez in attack, flanked by BBC's African Footballer of the Year Yacine Brahimi and the enigmatic Ricardo Quaresma, Bayern was unsettled. Such a high-line had danger written all over it - and it was almost thrown into complete disarray when Manuel Neuer committed the tackle that led to Porto's penalty kick.
In defense, Bayern lacked concentration, the first-touches of defensive players under pressure at the root of many mistakes. Alonso was a culprit, but equally Dante and Rafinha were caught out considerably with poor first touches that allowed Porto to execute their pressing tactics.
For the second goal, a heavy touch from Dante was the impulse for Quaresma to close down, force the mistake and roll the ball into the net basically unopposed. The third fell at the feet of Jerome Boateng who misjudged the high ball and Martinez's likewise was unchallenged as he beat Neuer.
Even with years of experience and a stackload of honors, Bayern were naïve and fell right into the trap of Porto's Basque coach Julen Lopetegui.
With a tight trident of classy midfielders who can move the ball with one-touch under pressure, Porto was able to resist Bayern's midfield pressing.
Oliver Torres, in particular, on a season-long loan from Atletico Madrid, delightfully evaded the controlled pressing in the middle of the park with some intuitive flicks to set Porto on the counterattack. Algerian international Brahimi completed seven take-ons against Bayern and was important in buying Porto time and space on the break.
What has been demonstrated for several months is now that Bayern Munich is extremely vulnerable to a well-organized pressing team. The defeat to Red Bull Salzburg in January last year appears a long time ago, but the same deficiencies remain and have been exasperated since the turn of the calendar year.
Bayern and Guardiola will need to sharpen up quickly if the German champions want to turn a routine title-winning season into a memorable treble-winning one.
Injuries have limited Bayern's attacking capacities, but defensively, there's no excuse. Pep will be demanding his defenders shape up, or will be left out.