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 Americans went into the clash with Belgium on a five-game unbeaten streak that created optimism but were missing several injured regulars.
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.
With the roof closed in an air-conditioned stadium, fans booed the water breaks in the middle of each half — which will take place at each World Cup match.
Belgium were missing striker Romelu Lukaku, midfielders Leandro Trossard and Hans Vanaken and goalkeeper Thibaut Courtois because of injuries and fitness issues.
The US will open World Cup play on June 12 against Paraguay, but coach Mauricio Pochettino is more concerned with the match against Portugal on Tuesday, also in Atlanta, which will be his final chance for evaluation.
After that, the US will not take the field again before the roster is announced on May 26. Three days later, the Americans play Senegal in Charlotte, with the final tune-up June 6 in Chicago versus Germany.
- with AAP and Reuters
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