I'm sorry, but up goes my bullshit flag again.
Invisibility. Come on. If you're telling me that you were training (I'm assuming you don't find yourself fighting Ninjas while you're out strolling), and your opponent struck you and moved, thereby giving YOU the impression that he is invisible, then I believe that. However, if you're telling me that a third person observing the fight would also not be able to see the ninja, then my bullshit flag must remain hoisted.
I'll give you an example of when I personally witnessed my opponent becoming invisible: I was a young recruit in basic training, and was preparing to face my opponent in a pugil stick match. I looked across the bridge at my opponent and figured I could take him. I won my last match. The loser of this match would find himself in the water, and I wasn't particularly eager for a bath.
Now, for those who are unfamiliar with pugil sticks, I'll explain a bit. You wear a football helmet and a pair of heavy padded gloves. You're given a long stick with thick, round pads at each end. The stick isn't particularly heavy, but it gets that way pretty quick. You attack your opponent using techniques you learned during your bayonet training, and attempt to slash, parry, thrust, smash, etc. Truthfully, the match always degrades to a slug-fest, with each opponent simply clobbering the other. I was pretty good at that.
Anyhow, my match went something like this: Cocky little me walks out onto the bridge with a bit of a smile on my face. I was going to enjoy this. My opponent walks out to meet me, and he looks intense - he's breathing heavily through his nose, and I think I even heard him growling. I almost laughed; I thought he was a bit TOO intense. What I wouldn't give to go back and re-evaluate that assessment! When the drill instructor blew the whistle, he struck first. I felt a blow to the head, and suddenly he's gone! The bridge was only a few feet wide, so he couldn't have gone anywhere, yet he was gone. I'm still crouched in my fighting stance, trying to make sense of what just happened, when I feel repeated attacks from the rear. The next thing I know, I'm tumbling into the water.
So what happened? It appeared this guy became invisible, and somehow snuck around behind me! In truth, nothing of the sort happened. He hit me so freaking hard with his first strike, that he spun me around (something about where the head goes, the body will follow...). I was so stunned, that I didn't even realize I was facing away from him. I learned a lesson that day, and it had nothing to do with invisibility - it had to do with preparation!
Ninja vanish? I don't think so. Quicker than you? More probable.
"The object of war is not to die for your country but to make the other bastard die for his."
General George S. Patton
"Ha, Ha. I've never seen anyone suck as bad at this as you!" SGT Anderson
"There's no duck! There never HAS been a duck!" Meme