The five-second regulation allows the ninja to...

-Possibly circle behind the knight and attack from behind.
-Ready any hidden weapons.
-Come up with a plan, since the assassin is the generally the one with the preparation time.
-Obtain an element of surprise.


It does not, however, allow the ninja to...

-Poison the knight's food. Because that wouldn't invoke too much interesting discussion :/
-Or set up, like, 20 traps.
-Or heat-shot the knight with a low-bow as soon as the battle starts.
-Or attack the knight in his sleep.

etc.

Define additional opening conditions for this encounter. Terrain? Weather? Time of day? Holy s*** give me something to work with here.

Because otherwise the ninja would just run. This would be a no-win situation for him if he fought openly.

Ah yes, "ring out" counts as loss. Let's just say moderately uneven plain with rocks, and some trees. Afternoon Evening, cloudy, fair temperature, and they meet in the center of an enclosed circular area with radius 50 yards.

Edit: Changed timie of day. Probably didn't give the ninja a significant advantage as I intended to, but eh.

Mafia FTW.

Throws a rock to distract said kinght,then said ninja assassinates
The knight.

I was thinking about that, but doesn't the knight have solid armor?
How would that work if the ninja can't break through the knight's armor?

I think several strategies might work here. The choices seem limited, though.

The ninja can somehow try to assassinate before the knight notices, whether that means using a rock to distract or sneaking up from behind. A heavy weapon should do the job here. However, using a piercing weapon might not result in a one-hit Ko, which can be a big problem for the ninja. Or, the ninja can try using a Kusarigama to immobolize the knight, then attack the fallen knight by lifting off the visor. (Both assume that the knight is clad in gothic plate with chain-mail underneath, and wears a closed helmet.)

Doesn't the closed helmet have several vertical slits in it?
A ninja's sword is thin enough to enter one of those slits.
Knight gets sword in the face

That probably would mean the ninja's going to enter melee combat with the knight head-on. Not sure he'd want to do that. Also some helmets use holes instead of slits.

Poor ninja.

In this fight, it seems like the ninja can only attack in succession far away.
Up close, I think the ninja has no advantages.

Sword through Head often kills.

the ninja throws a a smoke bomb.
during the confusion,he takes of the knights helmet and then...

Even if the ninja couldn't assassinate the night in the 5 seconds, he would still stand a good chance in a one on one battle. The ninja would be faster then the knight, meaning the ninja could probably dodge most of the knight's slow and powerful attacks. Not to mention the knight could probably only be on the ground, while the ninja could use tree's and good old ninja manipulation to escape and then attack the knight. As for a weapon, ninjas still have there short ninja blade that is fairly decent. The armor of the knight doesn't really matter all that much considering the only thing the ninja needs to do is knock off the knight's helmet and attack the head(which shouldn't be too hard, given the ninja'd superior speed.) Though I suppose if the knight got one good hit on the ninja, it would probably be over. In the end it really comes down to if the knight is skilled enough to land an attack on the ninja.

Be logical. You all seem to think taking off the knight's helmet would be no big deal, but that's simply not the case. Moreover, the armor would render any ranged attacks the ninja might attempt to utilize completely useless. That in turn means the ninja's superior agility is substantially less helpful considering he would have to close the gap to do any damage to the knight anyway. Without the ability to perform a stealthy, silent kill, the ninja is at a severe disadvantage. My bets are on the knight.