FBC reports Tonga dominated the scrums and line-outs and scored two first half tries for a 15-nil half-time lead.
Ben Volavola opened the scoring for Fiji with a solo effort in the second half, 15-7.
Nemia Soqeta, playing in his tenth test, added another try minutes later to close the gap to 15-12.
Fiji added two penalties while Tonga converted one for the scores to be leveled at 18-all with nine minutes to go.
Eremasi Radrodro scored late in the match which proved to be the winning try.