How do I follow up with such opener?

"I think I know you"
"I saw you"

If she ignores ...
If she asking "Where?" ...

I suggest something like:
I said "Saw your anime look on me back at the city center"
If she says "I wasn't there for a long time"
I respond "Kidding, but thats my impression of you"
or "Never too late to experience that with me, do you like blue tea?"