The five try to two victory was built around a strong first half from the Warriors who built a 21-9 lead at the break.
The Samoans fought back through tries to prop Howard Sililoto and halfback Melani Matavao but Fiji's captain Mosese Voka sealed the victory with a late score.
In Friday's early game Junior Japan were too strong for Tonga A, winning 45-28, thanks in large to a hat trick of tries from number 8 Tevita Tatafu.
