Angelo Padilla netted a second-half brace in Carmelita's 4-1 win away from home on Sunday, while Saprissa were forced to settle for a 1-1 draw at cellar-dwellers Uruguay.
Carmelita's victory moved them to nine points from four rounds, just a point adrift of Saprissa.
UCR (+4) and Cartagines (+4) are level on points with Carmelita (+6) but behind due to their inferior goal difference.
Belen Siglo XXI and Carmelita exchanged goals in the opening half, with Minor Diaz's ninth-minute penalty restoring parity for the home side after Pedro Leal's opener at the Polideportivo de Belen.
It was all one-way traffic in the second half, with Guatemalan attacker Padilla sparking the onslaught.
Padilla - recruited from Municipal at the start of the Torneo season - put the visitors ahead in the 66th minute before Ronney Mora opened up a two-goal lead with three minutes remaining.
Carmelita sealed the points in the third minute of injury time thanks to Padilla, who made no mistake from the penalty spot for his fourth goal in as many games.
At the Estadio Municipal El Labrador de Coronado, Saprissa's 100 per cent record came to an end after they shared the spoils with Uruguay.
Hosts Uruguay hit the front in the 10th minute thanks to Joshua Diaz, before Jordan Smith equalised for Saprissa five minutes into the second half.
In other results, Jonathan Sibaja's 56th-minute strike inspired UCR to a 1-0 victory over Limon.
Cartagines were also 1-0 winners, edging Perez Zeledon at the Estadio Jose Rafael Fello Meza.
Meanwhile, Santos de Guapiles claimed back-to-back wins with a come-from-behind 2-1 victory over strugglers Puma Generalena.