Germany came out aggressively, immediately putting Ghana under pressure with early chances. Nick Woltemade squandered a close-range opportunity, while Florian Wirtz rattled the post inside the opening 10 minutes.
They thought they had taken the lead through Wirtz in the first half, only for VAR to rule the effort out for offside. The breakthrough eventually arrived in stoppage time before the interval when Jonas Adjetey was penalised for handball.
Kai Havertz stepped up and calmly converted the penalty, giving Germany a deserved lead at the break.
Germany pushed again after half-time, with Woltemade heading against the crossbar, but their inability to extend the lead proved costly.
Ghana grew into the game and punished the hosts on 70 minutes, as Abdul Fatawu finished from close range after good work by Derrick Kohn.
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With the match heading towards a draw, Undav made the decisive impact. The Stuttgart striker struck in the 88th minute, reacting sharply to secure the win.
Despite his match-winning goal, Nagelsmann made it clear that Undav’s role is unlikely to change ahead of the World Cup.
“The roles are for the World Cup, not for March,” the Germany coach said. “He did what a goalscorer has to do. He can take chances when the opponent is tired.”
Undav responded: “If I keep scoring goals, maybe that will change.”
The forward also praised the home support but called for unity after some fans jeered Leroy Sane.
“Leroy is part of the team, just like everyone else. We should support everyone,” he said.
Ghana, now on a four-match losing run, will look to respond in their next friendly against Mexico, while Germany face Finland full of confidence.