der Tennisspieler
tennis player
A male person who plays the sport of tennis, whether as a hobby or professionally.
Alexander Zverev ist ein berühmter deutscher Tennisspieler.
Alexander Zverev is a famous German tennis player.

tennis player
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tennis player
A male person who plays the sport of tennis, whether as a hobby or professionally.
Alexander Zverev ist ein berühmter deutscher Tennisspieler.
Alexander Zverev is a famous German tennis player.

The word is a clear example of a compound noun. It consists of the sport 'das Tennis' (tennis) and 'der Spieler' (player), which is derived from the verb 'spielen' (to play).
Schon als Kind wollte er ein professioneller Tennisspieler werden.
Even as a child, he wanted to become a professional tennis player.
Combining a sport with '-spieler' is a very productive pattern in German for referring to people who play that sport. Further examples are 'Fußballspieler' (soccer player) or 'Basketballspieler' (basketball player).
Mein Bruder ist ein begeisterter Handballspieler in einer lokalen Mannschaft.
My brother is an enthusiastic handball player on a local team.
Common adjectives used with 'Tennisspieler' are 'professionell' (professional), 'berühmt' (famous), or 'erfolgreich' (successful). Typical verbs are 'trainieren' (to train), 'ein Turnier gewinnen' (to win a tournament), or 'gegen jemanden spielen' (to play against someone).
Der junge Tennisspieler trainiert jeden Tag mehrere Stunden.
The young tennis player trains for several hours every day.
player
'der Spieler' and 'der Tennisspieler' can both refer to a person who plays a sport or takes part in a game. 'Spieler' is the more general word and fits many games and sports. 'Tennisspieler' is more specific and refers only to someone who plays tennis. The words can be confused because a Tennisspieler is also a Spieler. If the context of tennis is clear, 'Spieler' can be enough. Without a clear context, 'Tennisspieler' is more precise.

Der Tennisspieler gewann das Match nach drei langen Sätzen.
The tennis player won the match after three long sets.
Beim Turnier bekam jeder Spieler eine Nummer.
At the tournament, every player was given a number.