The no-nonsense former mayor scoffed at questions from reporters about whether China's rapid reclamation of uninhabited reefs or an international arbitration ruling last year would be brought up with ASEAN leaders on Saturday.
"Who will dare pressure?" he told reporters at the presidential palace after meeting his counterpart from Brunei, Sultan Hassanal Bolkiah. "Who can pressure China? Us?"
The Philippines is hosting meetings of ASEAN this year. The bloc will adopt a softer than usual tone about South China Sea disputes and exclude references to militarisation or island-building, according to a draft of the chairman's statement.
The statement would be a watered-down version of that issued last year and comes amid a charm offensive by Mr Duterte, who has opted to court China for its business and investment and avoid rows over sovereignty.
Mr Duterte has been accused by members of the previous administration of taking a defeatist position on China and on defending Philippines sovereignty.
His predecessors in 2013 filed a case with the Permanent Court of Arbitration in The Hague to set the record straight on maritime boundaries. The tribunal did that last year, and invalidated China's claim to sovereignty over most of the strategic waterway.
Mr Duterte, who has put the ruling on the back burner and said he will revisit it later in his term, said it was a waste of time for ASEAN to discuss that award now, and it was not relevant.
"Arbitral is simply entitlement. It's not even a territorial thing. The only question at arbitral was entitlement, not jurisdiction, not even territory," he said.
"How will you raise the issue? .... We cannot on our own enforce the arbitral judgment."
(Reporting by Martin Petty and Manolo Serapio Jr, Editing by Simon Cameron-Moore)