If Al Hilal can do the same like playing against Manchester City, Fluminense won't be a threat for them advancing into semi finals. Chelsea at this level isn't the best opponent that can stop Al Hilal from advancing further, with their struggling performance the gap performance seems to be wide, Al Hilal started with Real Madrid, did the same to Manchester City while Chelsea were struggling against Benfica if not for the red card that helped them.
Brazilian players are very talented even they are not famous players and Al Hilal must play this match very carefully. They have more star players than Fluminese and their squad have higher value than Fluminese's squad, but all those things don't automatically bring any win to Al Hilal. Fluminese won't play this match without any ambition to win it, and they will play this match very seriously. Al Hilal will be challenged by Fluminese and actually have risk to lose it.
Chelsea has to beat Palmeiras tomorrow to decide their next opponent while Al Hilal vs Fluminense will be playing today.
Chelsea are still in building progress and they value this FIFA Club World Cup to win another title that can give them more confidence and good preparation for an next season. Generally Brazilian clubs are performing very impressively in this tournment and both Fluminese and Palmeiras have their opportunities of going next.