Howard was in terrific form from the first minute when he denied Divock Origi and he made a series of excellent stops.
Origi also hit the crossbar before U.S. substitute Chris Wondolowski missed a glorious chance to win it in stoppage time when he side-footed over the bar from point-blank range.
Belgium introduced substitute striker Lukaku for the extra period and he made an immediate impact, escaping down the right and crossing low for De Bruyne to show good control before slamming in the opening goal within three minutes.
De Bruyne set up Lukaku to drive in the second in the 105th minute and although American teenaged substitute Julian Green pulled one back two minutes later Belgium held out to complete a clean sweep of group winners in the last eight.
Image: Kevin De Bruyne of Belgium celebrates with teammates after scoring
Photograph: Robert Cianflone/Getty Images