Slovakia are just the fourth team to beat Germany in a World Cup qualifier
Germany suffered a surprise defeat away to Slovakia in their opening World Cup qualifier in Bratislava.
David Hancko gave the hosts the lead three minutes before half-time, running from his own half before playing a one-two with David Strelec, and finishing past visiting keeper Oliver Baumann.
Middlesbrough forward Strelec added the second when he bamboozled Antonio Rudiger 10 minutes after the restart with some tidy footwork before curling a shot into the top corner.
This was only the fourth World Cup qualifier Germany have ever lost.
Liverpool's Florian Wirtz and new Newcastle signing Nick Woltemade were both in Julian Nagelsmann's side, but the visitors were disappointing in attack.
Northern Ireland won 3-1 in Luxembourg in Group A's other game and visit Germany next on Sunday (19:45 BST).
The six four-team World Cup qualifying groups are starting this week, with six five-team groups having started earlier this year.
The top team from each group qualify for the World Cup, with the runners-up going into play-offs.
Elsewhere, Spain won 3-0 in Bulgaria in their opening Group E fixture with goals from Mikel Oyarzabal, Marc Cucurella and Mikel Merino.
In Group G, a late Matty Cash strike gave Poland a 1-1 draw away to the Netherlands, who opened the scoring through Denzel Dumfries.
And Belgium won 6-0 in Liechtenstein in Group J, with Wales beating Kazakhstan 1-0 earlier in the day.
