Saprissa had won five matches in succession - six in all competitions - prior to the weekend, but were humbled by lowly Belen Siglo XXI 4-3 on Sunday.
Jeaustin Campos' men appeared on track to preserve their winning run despite conceding in the second minute.
Mario Centeno put Belen Siglo XXI ahead early, but the Estadio Eladio Rosabal Cordero hosts were trailing 16 minutes later after goals to Saprissa's Keilor Soto and David Ramirez.
But Belen Siglo XXI's response was emphatic - Leonardo Adams equalising prior to half-time before a six-minute cameo from Jose Luis Cordero gave them a match-winning two-goal lead.
Ramirez's second of the match with six minutes of regulation gave the visitors hope, but they fell to their fourth defeat of the campaign.
Cellar dwellers Uruguay earned their first win of the season, Jonathan Moya's penalty on the stroke of half-time helping them beat second bottom Puma Generalena 1-0.
