Goals to Johan Condega and Andres Lezcano saw Cartagines defeat Deportivo Saprissa 2-1, leapfrogging the vanquished side into third in the standings and claiming an unassailable five-point advantage of fifth-placed UCR.
It means the Torneo de Invierno semi-finalists are confirmed heading into the final round of the regular season, which will be held on Sunday.
Condega's free-kick in the ninth minute put Cartagines on the way to victory, while Lezcano made it 2-0 three minutes after half-time, although he was given a helping hand by Saprissa's goalkeeper, who let the long-range strike slip through his fingers into the net.
Ariel Rodriguez pulled a goal back for Saprissa in the 88th minute but it mattered little as Cartagines claimed their fourth win in five matches.
The win took Cartagines to 39 points, one ahead of Saprissa, while UCR's (34) hopes of reaching the semi-finals are dashed.
Cartagines will face Herediano in the semi-finals but will have to play the second leg away from home after the latter wrapped up second spot on Wednesday.
Yendrick Ruiz struck a brace as Herediano destroyed Limon 4-1 to rise to 43 points, seven behind leaders Alajuelense, who will play Saprissa in the semi-finals.