Tatafu scored two tries for the Junior Japan in the first-half which were converted by nippy halfback Naoto Mano, who kicked two penalties that took their scores to 20 points.
Despite being deprived of possession for most of the first-half, Samoa A scored an unconverted try through lanky lock Talaga Alofipo.
Fiji Times reports Samoa A skipper and fly-half, Patrick Faapale kicked three penalties as Japan led 20-14 at half-time.
