Alajuenlense's unbeaten start to the Torneo de Invierno season came to an end on Wednesday, beaten 2-0 at Belen Siglo XXI.
Oscar Ramirez's Alajuelense had won nine of their opening 10 matches prior to kick-off, but second-half goals from Jose Cancela and Ariel Santana condemned the visitors to their first defeat of the campaign.
Alajuelense (+16), who have three games in hand on rivals UCR, remain at the summit on goal difference with 28 points after UCR (+11) were held to a goalless draw by Herediano.
Deportivo Saprissa continued their surge up the table, accounting for Santos de Guapiles 1-0.
Heiner Mora's 29th-minute strike was all that separated the two teams as third-placed Saprissa made it three victories on the bounce.
Saprissa are just three points adrift of Alajuelense and UCR.
In other results, Cartagines crashed to back-to-back defeats.
Beaten at Herediano last time out, Cartagines lost 1-0 away to Perez Zeledon.
Cartagines - fourth in the table - could not find a way back into the contest following Luis Perez's ninth-minute goal.
Puma Generalena snapped a six-game winless streak, downing Limon 1-0 courtesy of Davis Vergara.
Meanwhile, basement side Uruguay and Carmelita played out a scoreless draw.
