Sari spot kick sends Turkey past Germany in Berlin
Lightning-fast in attack, dogged in defence and buoyed on by a deafening crowd in the 'away' friendly, Turkey posed constant questions for Germany.
Kai Havertz gave Germany the lead after just five minutes but Turkey headed to half-time in front, after two late goals from Ferdi Kadioglu and Kenan Yildiz.
Borussia Dortmund striker Niclas Fuellkrug equalised shortly after halftime but Turkey would have the final say, Sari slamming in a penalty to secure victory.
"We were too lethargic, too passive, not purposeful enough and didn't move well in possession" Ilkay Gundogan, the first Turkish-German to captain Germany, told RTL.
"We didn't generate the pressure against the ball that we wanted," he added.
Visiting manager Vincenzo Montella's predictions of a "home" atmosphere were spot on, with the vast majority of the 72,592 fans at Berlin's Olympic Stadium drenched in red.
Around three million people of Turkish origin live in Germany, many in the capital.
The hosts' pre-match starting XI announcement was greeted with boos, as was every touch by anyone in black and white in the opening minutes.
Handed a start in the unfamiliar position of left-back, Arsenal forward Havertz repaid his coach's faith almost immediately, tapping in a Leroy Sane pass just five minutes in.
Havertz's goal failed to silence the Turkish fans, who were rewarded with a two-goal burst late in the first-half.
Abdulkerim Bardakci found Kadioglu with an expert long pass, the Fenerbahce midfielder hammering a brutal shot past a helpless Kevin Trapp.
Turkey's second was a carbon copy of the original, with a cross finding Germany-born Yildiz, whose powerful shot bounced off the crossbar and the post before landing in the net.
With Turkey on top, half-time came at the perfect moment for shellshocked Germany, who composed themselves to score immediately after the break.
Florian Wirtz carved through the middle of the field and found Fuellkrug, the Borussia Dortmund striker taking a touch before hammering in a low shot.
Fuellkrug, who made his international debut at the age of 29 after spending much of his career outside the first division, now has 10 goals in his 12 Germany appearances.
With 20 minutes remaining, VAR found a Havertz handball in the box. Sari stepped up to the spot and converted, sending the stadium into raptures.
The result, in Nagelsmann's first match on home soil and third overall game, shows Germany remain in transition just seven months away from hosting Euro 2024.
Germany have now won just three of ten matches since their group stage exit at the Qatar World Cup in 2022.
Germany had not lost to Turkey for more than 18 years -- and not on home soil since 1951.