South Africa opened the scoring in the second minute when Werner Kok went under the posts from close range.
Justin Douglas responded with Canada's only try against the World Series leaders, who finished the game with five tries.
Kwagga Smith and Senatla had tries before the break to give the South Africans a 12-point lead.
Senatla scored again three minutes into the second half.
The Canadians face the United States and Kenya later Saturday.