Oscar Ramirez's Alajuelense scored twice in the final six minutes to upstage fourth-placed Cartagines 2-1 in Torneo de Invierno action on Wednesday.
Alajuelense - beaten 1-0 at Herediano last week - are three points clear at the summit with 34 points from 14 matches.
After a goalless opening half at Estadio Alejandro Morera Soto, Cartagines striker Johan Condega broke the deadlock with 23 minutes remaining.
Condega's initial shot was saved by Alajuelense goalkeeper Patrick Pemberton but the Costa Rican frontman was not to be denied a second time around after turning his opponent and shooting from an acute angle.
And just as hosts Alajuelense appeared beaten, substitute Jonathan McDonald popped up with an equaliser from close range in the 84th minute.
Alajuelense completed the remarkable comeback five minutes later, Armando Alonso controlling and poking the ball beyond Cartagines goalkeeper Alejandro Gomez.
Saprissa, who have played an extra game, are three points adrift following their 4-1 demolition of Perez Zeledon on the same day.
Goals from Carlos Saucedo, Diego Estrada, Daniel Colindres and Marlon Escoe inspired the rout at Estadio Ricardo Saprissa Ayma.
In other results, UCR's winless streak extended to five games after they lost 2-1 at Santos de Guapiles.
UCR - third in the table - are five points off top spot, though they have played three more games than Alajuelense.
Strugglers Puma Generalena and Belen Siglo XXI played out a goalless draw.
Meanwhile, Herediano made it three wins on the bounce with a narrow 1-0 victory over Carmelita on Thursday.