Fizz cast zhonya's and I, as Soraka, put my silence pool right beneath him. As soon as his zhonyas ended, and this is during my silence duration, he cast playful trickster and hopped out of it.
Just to make sure, Zhonya's ended before your pool 'collapsed' and silenced him? Because Zhonya's can dodge the silence itself, and after Zhonya's is over, if the timing is not exactly right, he can E out of it.
Unfortunately, any field type spell does not work like "once a champion is within range, activate". Instead, it checks every now and then for whether or not a champ is within the field, and if there is, it would proc its effects. Though it can be as frequent as 0.1s, that is still a gap of time in which an action may be taken, and for something as instant as Fizz's E, it is something very possible. Same thing for double Zilean bomb right on top of someone with Zhonyas; if they just so happened to come out between the time checks, they can blink out without triggering the intended effects.
