Milo featured for 75 minutes and Siljak levelled for Slovenia after Prso gave the home side a fifth minute lead. Hostilities resume with home advantage for Slovenia on Wednesday.
Meanwhile, in the European Under-21 Championship play-offs, Helder Postiga's Portugal were beaten 2-1 at home by France.
Anthony Gardner will be hoping for a first England cap in the friendly against Denmark this afternoon, kick-off 4pm.
Milenko Acimovic's Slovenia gave themselves a great chance of Euro 2004 qualification by securing a 1-1 draw in the play-off first leg in Croatia.