The tzaddik guards the "covenant of the world" by revealing the inner point (nekuda pnimit) within every aspect of Creation; in withdrawing from creative acts of labor on shabbat, the Jewish People guard this covenant on a weekly basis.
The human being is physically created on Friday, then serves as the connector of the World to its Creator--and testifies to this spiritual connection by observing Shabbat.
The order of creation depends not on chronology but on activity.