Fellaini played the full 120 minutes for Marc Wilmots’ side in Salvador, with Januzaj an unused substitute, as Belgium won it through goals from Kevin De Bruyne and Romelu Lukaku to book a last-eight clash with Argentina.
Belgium had enjoyed the better of a frantic, exhilarating match at Arena Fonte Nova, from the moment Divock Origi drew a save from Everton keeper Tim Howard after two minutes.
USA were troubled by the pace of frontman Origi and winger Eden Hazard, with Belgium looking particularly menacing on the counter-attack. From one such break, Hazard’s former Chelsea team-mate De Bruyne mishit the Red Devils’ best chance wide when well placed just inside the box.
Fellaini was a nuisance at set-pieces as he sought to impose himself physically at both ends of the pitch, and looked set for a tap-in after 25 minutes, only for Jan Vertonghen to delay his cross too long.
The Americans’ best chance of the opening 45 minutes fell to Clint Dempsey but the forward’s sidefooted shot was easily claimed by Thibaut Courtois.
Belgium took a firmer grip of proceedings after the interval and, after Fellaini drove well wide from distance, Origi headed onto the top of the crossbar and Dries Mertens volleyed agonisingly wide.