Saprissa dominated the shot count at the Estadio Alejandro Morera Soto in Alajuela but were unable to breach the home side's defence, leaving the two-legged final evenly poised ahead of Saturday's second leg.
Alajuelense and Saprissa entered the first leg level on 29 championships apiece in Costa Rica's Primera Division, with this season's summer tournament - or Torneo de Verano - set to give one of the traditional rivals their 30th league title.
In another edition of the Clasico del Futbol Costarricense, Saprissa took 18 shots, while the hosts only managed nine.
Daniel Colindres, Hansell Arauz and Carlos Saucedo had the best chances to score for the visitors, while Jonathan McDonald worked hard up front for Alajuelense.
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