In March 2008, Šišić made his first and only appearance in the UEFA Champions League, coming on as a substitute in a 3–0 loss to Chelsea at Stamford Bridge in the first knockout stage of the competition.

Šišić found playing time difficult to come by under Valverde, and by the winter he had not made a league appearance.

During the winter transfer period, Šišić and Olympiacos decided to part ways, and the Slovenian signed with Serbian giants Red Star Belgrade Šišić made his league debut with Red Star in February 2009 in a derby against Partizan Belgrade.

In January 2010, Šišić returned to Greece, signing for Super League club PAS Giannina.

He totalled 15 caps[1] and his final international was an October 2012 World Cup qualification match away against Albania.