Pronounciation of "X" in the beginning (1)

You,English speaking people,pronounce the word beginning with "X"
as [z] e.g. Xerxes [za:rksi:z] , Xanthippe [zaenthipi] , Xavier[zaeivier].
Why [z], not[ks] ? But you can pronounce [ks] in the middle or last.
Is there anyone who can tell me the linguistic explanation of it ?

In Japanese , of course , we say [kserkses],[ksanthippe] ,but we call
the Saint [zavier] , who introduced the Christianity into Japan,
maybe because of his original sounds .

Regards,   Toshiyuki Saito , in the suburbs of Tokyo
------------------------
## TOSHIYUKI SAITO
## saitone@onyx.dti.ne.jp
######################