Kessie turned in from close range following strong work by teenage Ivorian phenomenon Yan Diomande, who is in the sights of some of Europe’s top clubs, including Liverpool.
The Germans thought they had found the net twice in the first half.
Nathaniel Brown’s corner in the 21st minute was knocked into the goal by Aleksandar Pavlovic, but Paraguayan referee Juan Gabriel Benitez immediately called it back – charging Pavlovic with fouling Ivorian goalkeeper Yahia Fofana.
Then, in the 38th minute, German forward and Arsenal star Kai Havertz had the ball in the net, but it was disallowed because of a foul by Bayern Munich’s Jamal Musiala.
Fifa World Cup schedule and results
When the second half began, the Germans applied relentless pressure.
The vaunted Ivorian defence held firm till the 68th minute, but ultimately the Germans broke through.
Undav, of VfB Stuttgart, volleyed in a cross by Nadiem Amiri in the 63rd minute.
His second sent Nagelsmann and the German crowd into a frenzy as the Germans snapped a string of disappointments at the last two World Cups.
“Winning mentality, team spirit – we had exactly what you need to be successful in a tournament,” Germany defender Jonathan Tah told MagentaTV.
“We never gave up, we kept going no matter what. The guys coming off the bench brought energy. Deniz deserves a special mention, outstanding!”
The Ivorians have never advanced at a World Cup, but may still move on with a win over debutants Curacao in their final group stage match.
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