The banners and signs — one of which declared, “Sinner for president.” Late in the second set of his victory over Taylor Fritz at the ATP Finals on Tuesday, Jannik Sinner used all the adulation in his home country to his advantage.

Taylor Fritz benefited from a questionable serving decision by Daniil Medvedev and frustrated his opponent so much that the Russian broke his racket and was docked a point in a 6-4, 6-3 victory for the American in the opening match of the ATP Finals.