Coach Oscar Ramirez appeared deflated as he spoke to reporters after the match in Samara, where Aleksandar Kolarov’s goal was enough to secure a Serbian victory.
Ramirez faces the task of rallying his players before they play five-time World Cup winners Brazil in St. Petersburg on Friday.
"I told the lads this isn't over and we have to continue to be ambitious," Ramirez said.
"It would have been very important to win. Maybe we have lost a certain margin, but we still want to qualify and move onwards."
"We will now play the second and the sixth-placed teams in the FIFA rankings, and it is going to be difficult. However, our intention remains the same."
Brazil sit second in the world rankings, with Switzerland, the fourth team in Group E, are ranked sixth.
Four years ago, Costa Rica secured shock victories over Italy and Uruguay in the group stages on their way to reaching the quarter-finals.
Therefore, they know they can cause an upset against one of football’s superpowers yet the loss to Serbia means replicating the success of four years ago looks unlikely.
"From what I have seen in the locker room, I think that we still believe in our opportunities," stressed Ramirez, who took over as head coach in the wake of the last World Cup.
"We know it is going to be hard because we have to face Brazil and Switzerland, and this would have been a good opportunity to have a buffer ahead of those games. But who knows? - Maybe we can get a hold of that Brazil game.”
"We are in the situation we are in, and we will do the best we can."
(Reporting by Jack Tarrant; Editing by Hugh Lawson)
