The Kingdom trailed 15-3 midway through the first half before scoring four straight tries to lead 29-15 just before the hour mark.
The South Americans were awarded a penalty try to cut the deficit to seven points with 22 minutes remaining and scored again two minutes into stoppage time but missed a conversion that would have levelled the scores.
The boot of Baby 'Ikale Tahi first five Filipe Samate proved the difference in the match as he opened the scoring with a penalty and converted three of Tonga's four tries.