After a goalless first half, the contest came to life when Saprissa's American midfielder Moises Hernandez was sent off, putting in jeopardy their unbeaten run this season.
However, typifying their fortune that sees them lying clear bottom of the division table, Belen Siglo XXI went from having a one-man advanatage to a one-man deficit in the space of seven minutes.
Roberto Rojas and Kendrick Pinnock were sent off three minutes apart, and Saprissa exploited their advantage beautifully, finding goals from Marlon Escoe and David Ramirez to earn the three points in Heredia.
Saprissa have 21 points from nine matches to be four points clear of closest challengers Alajuelense (17 points).
Uruguay earned their first victory of the season, beating Puntarenas 1-0 at home courtesy of a Marvin Angulo goal with 15 minutes to play.
Santos de Guapiles enjoyed a 2-1 road win at Carmelita, with Reimond Salas scoring the winner early in the second half.
Limon's purple patch was ended on Thursday, as they fell 2-1 at home to Herediano.
Previously, Limon had opened their winning campaign in some style, stringing together three consecutive victories once they broke their duck.
Herediano joined Alajuelense on points, trailing them on goal difference but with a game in hand on both them and Saprissa.
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