Why is it der Winner?

Winner ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] Sport, mostly tennis: blow and point that decides the game

[1] Sport, meist Tennis: Schlag und Punkt, der das Spiel entscheidet

Examples

[1]

[1]

Declension

1 Singular Plural
Nominativ der Winner die Winner
Genitiv des Winners der Winner
Dativ dem Winner den Winnern
Akkusativ den Winner die Winner

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary