Goran Ivanisevic is starting six months military service in Croatia.
The 30-year-old has been able to put it off until now because of his tennis career.
The Wimbledon champion has been given permission to take time off for tournaments and will be allowed to take off his uniform to play in them.
Otherwise he will be treated like everyone else.
Military service is compulsory for all men of a certain age in Croatia.
Goran, who is engaged to model Tatjana Dragovic, will undergo 15 days basic training in the Lora army barracks in his hometown of Split.
As he walked in through the sentry post he said: "At least they have promised me I won't be taking part in any fighting. I prefer my battles on the court, and, to be honest, I doubt if I could even hit an elephant if they gave me a gun - I can't shoot at all."
Goran will be part of the Croatian army's special sports unit alongside three other Croatian Olympic medal winners.