The Germans, who have set a goal of winning the 2026 World Cup, had never before lost a World Cup qualifier on the road, and they have now lost their last three consecutive matches, following defeats by Portugal and France in the Nations League in June.
The Germans have nothing to show for their appearances in major international tournaments since winning the World Cup in 2014.
Slovakia's attacking style paid off in the 42nd minute when Germany's Florian Wirtz lost possession and the hosts launched a quick break with David Hancko completing it with a clinical finish.
Germany, toothless in the first half, looked more aggressive after the break, with Leon Goretzka coming close. However, Slovakia struck against the run of play, with David Strelec outmanoeuvring Antonio Rudiger before curling a shot past Baumann in the 55th minute.
Lothar Matthaus said after the match that he had been appalled by the German performance:
"The performance of the German team completely surprised me, almost shocked me. When goalkeeper Oliver Baumann was still the best man after a 2-0 defeat, it speaks volumes about the performance", said Matthaus to German media "Bild".
"Concentration, grit, passion - that was all missing. Germany allowed itself to be surprised by a tactically well-adjusted Slovakian team. The German team didn't have their head on the pitch."
In the days after the game, German media have discussed whether Julian Nagelsmann needs to field other types of players with less quality but who are willing to leave their heart out on the pitch, which Nagelsmann also spoke about himself after the game.
Matthaus also sees it this way: "Two weeks ago, I made a top eleven in which there were more players who had the right mentality. I'm thinking of (Robert) Andrich, (Waldemar) Anton, and (Nico) Schlotterbeck, too, when he's fit. These are mentality players."
Nagelsmann also received criticism from Matthaus due to his many experiments with the team.
"The tactics and formation surprised me. I think the basic formation should be a 4-2-2-2, just like Nagelsmann used to play in his days at RB Leipzig. He has a lot of tactical changes, shifts during games, and new processes all the time. I can't expect (Nnamdi) Collins to play well in his first international match when he has to change completely and things are required in a system that he doesn't normally play in (Eintracht) Frankfurt."
"Of course, you can't explain a defeat against Slovakia solely with tactics, but that doesn't give the players any security. Perhaps Nagelsmann will overwhelm one or two players with his ideas."
Germany continue their World Cup campaign when they face Northern Ireland at home on Sunday night at 20.45 CET.