De Ketelaere gave Belgium the lead in the ninth minute with a simple tap-in, taking full advantage of a nervous start by the US to quickly deflate the partisan crowd.
Malik Tillman equalised in the 31st minute with a deflected free kick which wrong-footed Thibaut Courtois, but De Ketelaere restored Belgium's lead little more than a minute later with a back-post header.
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The hosts played with more authority after the break until a calamitous error from goalkeeper Matt Freese allowed substitute Hans Vanaken to score in the 57th minute, irreversibly shifting the momentum in a stunned Seattle Stadium.
Romelu Lukaku then added a fourth in added time to send US fans streaming for the exits.
