Costa Rican defender Heiner Mora found the back of the net six minutes from time as Saprissa edged Carmelita 1-0 at the Estadio Ricardo Saprissa Ayma.
The result not only extended Saprissa's unbeaten streak to 16 games but sent Ronald Gonzalez Brenes' men three points clear of Herediano at the summit, while Carmelita remain fifth in the standings.
Herediano suffered a 1-0 loss at home to Belen Siglo XXI on Thursday.
Saprissa were held to a disappointing draw by lowly Puntarenas last Sunday and they appeared destined for another stalemate after a goalless opening half.
But with the match petering out for a draw, Mora popped up to score the match-winning goal in the 84th minute - his first goal of the season.
Saprissa have not tasted defeat since the opening day of the campaign, when they were beaten 2-1 by Perez Zeledon.
In Friday's other result, Alajuelense closed to within five points of Saprissa after a rampant 3-0 triumph against Uruguay.
First-half goals from Juan Guzman and Jerry Palacios, and an own goal from Seemore Johnson handed Alajuelense maximum points at the Estadio Alejandro Morera Soto.
Alajuelense still occupy third position after their third consecutive league victory.
Midfielder Guzman gave the hosts the ideal start, scoring on the half-hour mark before Palacios capped a dominant opening with a goal of his own on the stroke of half-time.
Alajuelense put the icing on the cake, albeit courtesy of an opposition player three minutes from the final whistle.
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