Castano, who was the 2005 runner-up, rallied to beat France's Aravane Rezai, 6-7 (9-11), 6-1, 6-3.
Romina Oprandi of Italy knocked off fourth-seeded Iveta Benesova of the Czech Republic, 7-5, 6-4; fifth-seeded Lourdes Dominguez Lino of Spain beat qualifier Lucie Hradecka of the Czech Republic, 7-6 (7-5), 6-4; and qualifier Eva Birnerova of the Czech Republic downed Kaia Kanepi of Estonia, 6-2, 6-4.
In the quarterfinals, Castano will play Israel's Anna Smashnova, who beat her in last year's final; second-seeded Michaella Krajicek of Netherlands faces Sara Errani of Italy; Dominguez Lino will play Oprandi and Birnerova goes against Martina Muller of Germany.
The tournament's eventual winner will receive $22,900.
Swim Week Miami Beach 2014 [PHOTOS]