Thursday's clash in Kaliningrad against England, who can assure their own qualification by beating Panama on Sunday, will stretch a side that truly believes it can go all the way in Russia and better a previous best semi-final finish.
"We can still do better," said the Manchester United forward, whose four goals in two games beat a previous Belgian record for scoring in a single World Cup finals and matches the current tournament leading scorer, Portugal's Cristiano Ronaldo.
"It is important we did better than in the previous match. That is what we did," he added. "Against England we still have to do better."
Already by far Belgium's top scorer in all international competitions with 40, at just 25 Lukaku is carrying high hopes that he is the final piece in a jigsaw -- providing the clinical finish that intricate build-up play from the likes of Eden Hazard and Kevin De Bruyne has provided in recent years.
"Today all the offensive players were able to bring something and the defenders were there too. As a team we keep improving," he said, playing down his own prospects of claiming World Cup records.
Replaced on the hour after nursing a bang to his heel, Lukaku said the injury did not seem too serious: "I could have gone off right away but I wanted to push it. We'll see with the physio tomorrow."
For coach Roberto Martinez, a couple of goals for Hazard and one for stand-by striker Michy Batshuayi mean a knock for Lukaku should not be too worrying.
Under the Spaniard, Belgium's "golden generation" are finally coming together after two disappointing quarter-final exits at the last World Cup and Euro 2016.
"It's not easy to score the amount of goals we did in two games," he said, recalling an opening 3-0 defeat of Panama.
Belgium have never scored five in a World Cup game before.
"It's really good to see a team coming together and wanting to fight so much for each other and then on top you've got the talent and the quality," added Martinez.
"It's been a joy since the day we came together to see a group of players working so hard for each other.
"We were in full flow going forward, we looked really sharp."
(Reporting by Alastair Macdonald ; @macdonaldrtr; Editing by Christian Radnedge)
