Jerry Palacios scored a brace for Alajuelense as they brushed past Uruguay 3-0 for their ninth win in 10 Torneo de Invierno matches.
Oscar Ramirez's men have played three fewer games than second-placed UCR, who slipped up at Puma Generalena.
Alajuelense needed just 35 minutes to take complete control at the Estadio Alejandro Morera Soto on Sunday.
Palacios struck either side of Armando Alonso's 17th-minute goal as Ramirez's side claimed a convincing win over the league's bottom side.
UCR coughed up a lead twice in a 3-3 draw at Puma Generalena.
Jonathan Sibaja and Ariel Contreras had the visitors in a dominant position by the midway point of the first half.
Anthony Calvo gave the hosts hope before Christian Bermudez equalised just after half-time.
Parity lasted a minute before UCR went ahead once more through Josue Martinez, only for a Jason Scott own goal moments later to complete the scoring.
Fourth-placed Deportivo Saprissa claimed their third straight league win as Ariel Rodriguez's brace saw them past Limon 2-0.
Yendrick Ruiz's goal was enough for Herediano to edge Cartagines 1-0 and Santos de Guapiles struck three times in the first half of a 3-0 win at Carmelita.
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