Armando Alonso scored an 81st-minute winner as Oscar Ramirez's Alajuelense edged fourth-placed Herediano 1-0.
With Deportivo Saprissa held, Alajuelense moved clear after their 12th win in 16 outings.
Herediano striker Victor Nunez had a chance to give his team the lead from the penalty spot, but his poor spot-kick was kept out by Patrick Pemberton.
Alajuelense rattled the post before Alonso struck with a 30-yard stunner to see his team to victory.
Deportivo Saprissa slipped six points behind after a 1-1 draw at Perez Zeledon.
Ariel Rodriguez cancelled out Fabrizio Ronchetti's first-half opener.
Santos de Guapiles were 2-1 winners at Puma Generalena, while Limon's clash against Uruguay was postponed.
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