I would argue that this is not an assumption. Something exists; we know that something exists; and we know that we know. What existence “is”, what knowledge “is”, how and why knowledge is possible—those are challenging questions. But doubting that anything exists, and doubting that there is any knowledge, seems to require willful negation of fundamental phenomenological facts.
And it’s not far from the existence of knowledge to the existence of “evidence”, since evidence is just, any fact that has implications for the truth; and it is part of the manifest nature of knowledge, that it comes via awareness of facts.
I like Ayn Rand’s related formulation: “Existence is identity, and consciousness is identification”. To be is to be something, and to be aware is to know something.