Keera

I would say that standing directly in front of an experienced adventurer, even while invisible, in preparation to attacking them, would be a bad idea.  Even though their eyes won't perceive you, their senses of hearing and smell could reveal that you are there, as well as that wonderful "6th sense" that lets someone know when they are being watched.  All it takes is one suspicious swipe with a weapon, and your done for!

When striking from behind, it's not such a problem - the person is bound to try to spin around before striking, and that gives you time for an attack, even if the target is suddenly aware of an attacker in their personal space.