This is the 906th most frequent Croatian word.
natjecanju
"Natjecanju" means "competition" (dative/locative singular form).
Here, 'natjecanju' is again in the locative case, referring to the specific competition as the context for his success.
Ponosan je na svoj uspjeh na natjecanju.
He is proud of his success at the competition.
The word 'natjecanju' is used in the locative case, referencing the activity or event (competition) being discussed.
Sudjelovao sam na natjecanju iz matematike.
I participated in a math competition.
In this instance, 'natjecanju' is used within a prepositional phrase to indicate the subject of the discussion (the competition).