The Beijing bronze medallists and European champions came into the Olympic Games as favourites, and their form will worry the other teams in the quarter-finals.
Slobodan Nikic and Stefan Mitrovic bagged hat-tricks for Serbia, while Andrija Prlainovic netted a brace.
Zivko Gocic, Dusko Pijetlovic, Filip Filipovic and captain Vanja Udovicic scored one each.
Romania's Kalman Kadar, Andrei Iosep, captain Cosmin Radu and Alexandru Matei fired home one apiece.
Serbia will play Australia in a quarter-finals clash on Wednesday.