Atletico lead the Spanish top flight by four points going into their final four games, the first of those being a trip to the Mestalla.
Diego Simeone's men are also in with a chance of reaching the UEFA Champions League final, having drawn 0-0 with Chelsea in their semi-final first leg on Tuesday.
However, on-loan goalkeeper Courtois is eager to focus on the test of facing Valencia, who have lost just one of their last six league games.
"As we are going game by game, now our biggest game is in Valencia," he told the club's official website.
"We are at a very good physical level. I think we can make one more effort on the pitch and that's good for the remaining games. Our first final is on Sunday."
While Courtois will have been pleased to deny his parent club an away goal in Tuesday's first leg, Valencia go into the game off the back of a 2-0 defeat to Sevilla in the UEFA Europa League on Thursday.
Courtois feels Valencia's schedule of playing twice in four days could play a big factor in Sunday's game.
"The good thing for us is that we have two more rest days because Valencia played on Thursday," he added.
"We should be going there with intensity because (playing at) the Mestalla is always a tough game.
"You have to play it as a final and only if we do so can we move forward."
