Arias netted from the penalty spot after Marcelo was fouled inside the box 19 minutes before the final whistle, and Kennedy later sealed their advancement with a second goal. The victory didn't come easy, as Arias faced some initial frustration, hitting the woodwork twice in the first half.
The breakthrough came when Arias confidently converted the penalty after Percy Tau brought down Marcelo. Tau had an opportunity to redeem himself immediately, connecting with a long pass from Hussein El Shahat, but his header went straight to Fluminense's goalkeeper, Fabio.
Fabio was called into action again, making a crucial save to deny Taher Mohamed, while at the other end, Mohamed El Shenawy kept Al Ahly in the game with a key save against German Cano in a one-on-one situation.
Fluminense's second goal arrived shortly after, with Kennedy, the hero of their recent Copa Libertadores triumph, calmly slotting the ball into the bottom corner from inside the area.
Despite El Shenawy's efforts to thwart Cano again, Al Ahly couldn't change their fortune, missing out on the chance to reach the Club World Cup final for the ninth time.
Fluminense now has the opportunity to become the first non-European team to win the tournament since Corinthians in 2012. Their opponent for the final will be determined after the Tuesday match between Manchester City and Urawa Red Diamonds.
