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Second-half substitute Romelu Lukaku added Belgium’s final goal in the third minute of stoppage time.
The US has a chance to advance to the World Cup quarterfinals in Seattle tonight.
Emerson Hancock (6-4) gave up two hits and two walks while striking out five over seven innings.
The US will have to continue its historical World Cup run without one of its top players on Monday.
The US is playing Bosnia-Herzegovina at 5 pm with a chance to advance to the Round of 16 in Seattle.
Bryan Woo (7-6) allowed just four hits and struck out five in 6 1/3 innings.
The Bells will look to secure one of three playoffs spots left in the North Division over the next month.
The official Fan Zone watch party at the waterfront starts at 1 p.m. with a Seattle-hosted match before the U.S. plays at 5.
Dominic Canzone also went deep to help Seattle (43-43) get back to .500.
The Mariners have fallen below .500 and out of the division lead for the first time in over a month.