Victory in race one in Sweden gave Fiva a lead of 140 points at the top of the standings and, despite making way for Jean Frederic Chapuis to take the win second time out, he now cannot be beaten.
Chapuis placed first in the big final ahead of Armin Niederer, Sylvain Miaillier and Filip Flisar while Fiva won the small final for fifth overall and the ski cross World Cup title.
With the season finale in nine days time in Sierra Nevada, Fiva, who placed third in the overall ski cross World Cup standings last year, holds a lead of 105 points over Swiss teammate Niederer.
Smith meanwhile recovered from not even record a placing in race one in Are following a failure to finish qualification to take a victory that all but guarantees her the ladies' World Cup title.
Smith beat Marielle Thompson, Swiss teammate Katrin Mueller and another Canadian Georgia Simmerling in the big final in Sweden for her fourth win of the season but first since December.
She now has a lead of 89 points over Ophelie David of France, who won the small final for fifth, at the top of the overall World Cup standings with one race and just 100 more points on offer.