During his teenage years, he grew 20 cm (8 inches) in an 18-month period, leading to knee problems that sidelined him for several months early in his pro career.

At Slovan he developed into a promising big man, soon making the (regular) Slovenia junior and under-20 national teams.

His statistics in the Adriatic League were arguably more impressive, considering that he played only 23 minutes per game in that competition; he averaged 13.2 points and 3.9 rebounds and also shot 47.3% from three-point range.

[8] Zupan signed with Union Olimpija, also of Ljubljana, in the 2006 offseason, fulfilling what he called one of his lifelong dreams.

He also barely missed out on a trip to the 2006 FIBA World Championship, becoming the last player cut from the Slovenia squad.

[4] His chance to play for Olimpija in the Euroleague was on hold for several months because his transfer became the subject of litigation between them and Slovan.

Olimpija had registered him at the start of the 2006–07 season, and he played six fixtures in the Adriatic League before Slovan contested the registration.

[9] Olimpija won the first round of the legal battle in August 2006, but on October 23, an appellate court set aside the decision and ordered a new trial.