Without question the turning point of this incredible drama is the court scene in Act IV scene 1 and the sudden reversal of fortunes that Antonio and Shylock both experience. Consider the way that the drama leads up to this climactic scene: the antipathy between Shylock and Antonio is made evident, and the "merry bond" that Antonio agrees to is finalised. Meanwhile, Shylock suffers the loss of his daughter, that makes his hunger for vengeance against the wrongs that he and his race have suffered all the more acute. Also, Antonio himself has suffered bad tidings, with news of the shipwreck of some of his ships making him unable to repay the bloodthirsty Jew.
As the court scene begins, we hold little hope for Antonio. Even Bassanio with all his new-found wealth is able to do little for his beloved friend. However, it is Portia that is able to bring about this situational irony, for suddenly it is Antonio who has Shylock in his power rather than the reverse. And Antonio is not slow to exert this power over his enemy:
Two things provided more: that for his favour
He presently become a Christian;
The other, that he do record a gift
Here in the court of all he dies possessed
Unto his son Lorenzo and his daughter.
The power of Shylock is broken after this scene and loss of power, and he transforms from an avenging, bloodthirsty figure determined to right the wrongs that he and his people have suffered to a pitiful figure, forced to convert to a religion that he detests and to leave his money to a daughter that has abandoned him and his religion.