Chelsea striker Lukaku will play a key role at the upcoming FIFA World Cup for coach Marc Wilmots after fellow forward Christian Benteke was ruled out with a serious Achilles injury.
And he warmed up for the Brazil showpiece in style, as his well-taken treble highlighted Monday's win in Genk.
Januzaj's appearance was also noteworthy, with the Manchester United youngster – the subject of lengthy debate about his international future – eventually settling the matter after coming off the bench at half-time.
Another substitute, Tottenham's Nacer Chadli, scored the fourth before Kevin De Bruyne netted a late penalty as Belgium, eventually, won well.
Goalkeeper Simon Mignolet sustained a minor injury in training on Saturday and Atletico Madrid's Thibaut Courtois was rested after featuring in the weekend's UEFA Champions League final.
With Hoffenheim's Koen Casteels also injured, 28-year-old shot-stopper Sammy Bossut – who plays his club football for Zulte-Waregem – made his international debut.
Fellaini headed a corner over before Lukaku got his second with 23 minutes gone, as he pounced after Maxime Chanot's attempted clearance fell into his path, and scored, via a ricochet off Moris.
Lukaku lifted over Moris – but wide – and his search for a treble went on as the goalkeeper saved superbly to his right three minutes before the break.
Januzaj was one of Wilmots' five half-time changes and saw Lukaku become the first player to score a hat-trick for Belgium since Bob Peeters achieved the feat in 2001.
