Zeros have no chance against a P-51 if the Mustang has the edge in E and/or altitude, all they can do is evade and hope you make a major blunder.
It's what I call the "51-Zero Dilemma".
Assuming average level of pilot skills, and both pilots with that same skill level, a P-51 will always DOMINATE the fight, except that final coup-de-grace just never comes out. In the Zero pilot's perspective, he basically has no hopes of turning the odds around, and yet, he finds it not too difficult to dodge and evade the P-51.
Both pilots will be just dancing around, each hoping the other guy makes a mistake, and typically only when the third guy belonging to any of their team intervenes, is when this silly stalemate ends.
Thus, when assuming 'normal' level of pilots, decent but not very skilled, for any of the two to shoot the other guy down, it takes a plane of some comparable characteristics to do so.