"I've come to realise that there are more important things than victories and defeats," the 33-year-old told a news conference organised by the Swiss champions.
Frei, who ended his national team career shortly after a goalless draw against Bulgaria in March last year, scored 42 goals in 84 appearances.
Frei's dreams of leading Switzerland to success as they co-hosted Euro 2008 were shattered when a knee injury forced him to limp out of their opening match with the Czech Republic before half-time. The sight of Frei wiping tears from his captain's armband was a poignant moment of the tournament
At Euro 2004, he was banned for three games on video evidence for spitting at England's Steven Gerrard and, in remorse, adopted a llama at Basel zoo - the Andean pack animals spit a foul-smelling substance at aggressors when provoked.
