der Finalist
finalist
A person or team that has qualified for the final round (the final) of a competition.
Die beiden Finalisten spielten um den Titel des Schachweltmeisters.
The two finalists played for the title of world chess champion.

finalist
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finalist
A person or team that has qualified for the final round (the final) of a competition.
Die beiden Finalisten spielten um den Titel des Schachweltmeisters.
The two finalists played for the title of world chess champion.

'Der Finalist' (the finalist) is derived directly from 'das Finale' (the final). The ending '-ist' refers to a person who takes part in a particular activity — here, in the final.
Nur der beste Spieler wird zum Finalisten gekürt.
Only the best player is named the finalist.
This noun belongs to the N-declension. This means it receives the ending '-en' in the accusative, dative, and genitive singular, as well as throughout the plural. A common mistake is to forget this ending.
Der Reporter interviewt den Finalisten.
The reporter interviews the finalist.
Common expressions are 'sich als Finalist qualifizieren' (to qualify as a finalist), 'die Finalisten stehen fest' (the finalists have been decided), or 'gegen den anderen Finalisten antreten' (to compete against the other finalist).
Er hat sich als erster Finalist für das Endspiel qualifiziert.
He qualified as the first finalist for the final match.
participant
'der Teilnehmer' and 'der Finalist' can both refer to people in a competition or event. 'Teilnehmer' is the general term: a person is taking part. 'Finalist' is more specific and means that this person has reached the final. So every finalist is a participant, but not every participant is a finalist. The words can be confused if you are only thinking about a competition. The important point is: 'Teilnehmer' only says that someone is taking part; 'Finalist' says that someone has made it especially far.

Nach drei Runden stand Max als Finalist des Wettbewerbs fest.
After three rounds, Max was confirmed as a finalist in the competition.
Jeder Teilnehmer bekommt vor dem Lauf eine Nummer.
Each participant gets a number before the race.