-2 win Wednesday night.
Sporting KC (9-4-5) has lost two straight and trails FC Dallas by three points in the West.
RSL (8-8-2) led 3-1 at intermission, but Sporting KC pulled within one when Ike Opara scored on a cross by Johnny Russell in the 64th minute.
Tim Melia kept the Sporting KC comeback alive by denying Albert Rusnak's penalty kick in the 80th minute, but Daniel Salloi was sent off in the 90th minute and Roger Espinoza earned a red card in the fourth minute of stoppage time.
Luis Silva scored on a penalty kick in the 10th minute of extra time.