Portia literally saves Antonio's life from Shylock's knife.  She is the one who studies Venetian law and the bond thoroughly and then develops the "loophole" which not only spares Antonio but that also severely punishes his enemy, Shylock.

Neither the Duke nor Antonio know that Portia is a female during this act (Act 4) because she is disguised as a young lawyer; Antonio would not know her connection to his friend Bassanio or even her motivation for saving him.  To the Duke and Antonio, it appears that she simply presented an objective view of the bond.