Two goals in the final 20 minutes in Jeddah by Jhon Arias, from the penalty spot, and then John Kennedy carried the Brazilians into Friday's final against European Champions League winners Manchester City or Japanese side Urawa Reds.
Al Ahly were left to rue a number of missed chances as the African champions failed to find a way past 43-year-old Fluminense goalkeeper Fabio. But he was not the only veteran to shine for the Copa Libertadores champions.
At the age of 40, Felipe Melo became the oldest outfield player in Club World Cup history, while ex-Real Madrid captain Marcelo, 35, won the decisive penalty. Former Brazil international Ganso, 34, and striker German Cano, 35, also played their part.
"Did the age of our players help? I think it absolutely did. Marcelo was decisive, especially for the penalty," said Diniz, who has combined his roles as Fluminense boss with a spell as interim Brazil manager.
Marcelo also believes it is a mix of youth and experience that has led Fluminense to an all-time high this year as they won the Copa Libertadores for the first time.
"They call us the 40-somethings, but we aim to help the younger players with this experience. I think it is this mix of young players and experience that has helped Fluminense a lot," said the left-back.
The last 10 Club World Cups have been won by European sides. But if the gulf is growing between Europe's elite and the rest of the world at club level, the gap between South America's best and the rest of the world has narrowed significantly.
Twice in the past three years, the Copa Libertadores winners have failed to qualify for the final and Fluminense could easily have followed suit. Al Ahly had dumped out a star-studded Al-Ittihad featuring Karim Benzema, N'Golo
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