Belgium exposed US defensive difficulties in Saturday's friendly in Atlanta, rallying from a late first-half deficit on two goals by Dodi Lukébakio and one each from Zeno Debast, Amadou Onana and Charles De Ketelaere.
The Americans are in Group D along with the Socceroos for the World Cup, with the two teams set to meet second up on June 19 in Seattle.
The US, missing several injured regulars, entered with a five-game unbeaten streak and coach Mauricio Pochettino's message to players had been they can win soccer's biggest prize for the first time.
Star striker Christian Pulisic failed to convert several good scoring chances. He entered scoreless in his last 12 games with AC Milan and extended his US goal drought to seven games since November 2024.
Pochettino was far from happy with his team's approach post-match.
"We cannot arrive with the wrong idea of that we are so good, we are so handsome, we are so well dressed and we are Americans," he said.
"Always after the games when the team wins, (it's) because we fight, we were aggressive, we were intense, yes? When we drop in that intensity — we saw today."
Weston McKennie put the US ahead in the 39th minute with his first in three years, but Debast and Onana began Belgium's rally on a tough afternoon for Matt Turner, the former No.1 American goalkeeper who made his first appearance since last June.
Debast scored in the 45th, Onana put the Red Devils ahead in the 53rd and De Ketelaere converted a penalty kick in the 59th after a hand ball by US captain Tim Ream.
Lukébakio entered in the 62nd minute and beat Turner from long range in the 68th and 82nd minutes to build a 5-1 lead before a disappointed largely pro-American crowd of 66,867 at Mercedes-Benz Stadium, site of a World Cup semi-final in July.
Patrick Agyemang scored a late consolation goal for the US in the 87th after Ricardo Pepi took advantage of defensive misplay.
Belgium extended their unbeaten streak to 10 games and have won six straight against the US since losing to the Americans at the initial World Cup in 1930.
The US will open World Cup play on June 12 against Paraguay, but Pochettino is more concerned with the match against Portugal on Tuesday, also in Atlanta, which will be his final chance for evaluation before his World Cup roster is announced.
- with AAP and Reuters
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