Sporting is now without a win in five straight games and falls behind DCU in the table.
The home team took the lead in the 28th minute with a fantastic Fabian Espindola goal from range. The United forward had the ball wide of the goal and outside the area when he spotted Sporting keeper Eric Kronberg off his line and sensationally picked out the far top corner with a shot.
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United didn't come out of the game fully unscathed however, as a reckless stoppage time challenge on Kevin Ellis saw sub Alex Caskey shown a red card.
D.C. leapfrogs Sporting to take second place in the East with a 6-4-3 record. Slumping Kansas City is now 5-5-4 and hasn't won since May 10.