However, Sporting's Bernardo Espinoza earned himself two yellow cards in three minutes just before half time, with the second awarded for a handball inside the box.
Roberto Trashorras dispatched the penalty into the bottom left corner to give Rayo the lead heading into the break.
Summer signing Javi Guerra then made it two early in the second half, pouncing on a rebound to slot the ball past helpless Sporting goalkeeper Alberto Garcia Cabrera.
The 10 men replied four minutes later, with Jony stealing the ball from the Rayo defence to run at the goal unchallenged before beating Tono one on one.
Manucho had a chance late on to extend Rayo's lead after a quick counter-attack, but the Angolan international dragged his shot wide.
The win lifts Rayo into eighth in the table while Sporting remain in 11th.