There could be no more worthy champions. The Rio de Janeiro giants made it through to their third final in four years and dominated all comers. They finish this campaign with 12 wins and a single draw. Their superiority was not in doubt.
But one of the great aspects of football is that the best side does not always win. With a bit of luck and a sound strategy, the underdogs can have their day. And Athletico could certainly count on the second of those elements.
In perhaps the last big game of a long career, Luiz Felipe Scolari was out to become the first coach to win the Libertadores with three different clubs. He knew that the key to success lay in maintaining Flamengo at bay, and he came up with a plan which caused an early surprise.
His selection looked like an orthodox back four. But instead Athletico were marking man for man over most of the pitch. Left back Abner Vinicius became a man marker in defence, along with Pedro Hernique, whose centre back partner Thiago Heleno was the free man.
Stop them, and Flamengo's creative flow can be halted, and in the early stages Athletico's plan could hardly have worked better. All that was missing was a goal snatched on the break. It nearly came when winger Vitinho cut across to take advantage of a David Luiz mistake, and hooked in a shot that was beaten away by Santos in the Flamengo goal.
t took Flamengo around half an hour to find any kind of flow. They started to rotate their players in midfield and attack, confusing the man-to-man marking and opening up space.
But they had little to show for it. Approaching the halftime whistle Flamengo had enjoyed 70% of the possession without posing any threat, without managing to exchange their customary quick passes around the edge of the area. Two questions hung over the Athletico effort.
In hot conditions could they be able to maintain the intensity of their marking for the full 90 minutes? And could they avoid the possible problem of picking up yellow cards?
The answer to the second question was an emphatic 'no.' The game changed shortly before half time when, already on a yellow, Pedro Henrique lunged into a rash challenge and was sent off.
Games can turn on decisions. One was Argentine referee Patrico Loustau's option to brandish the red card. Another was Scolari's delay to replace Pedro Henrique. There was a long hold up before the distraught defender left the field, giving Athletico time to reorganise.
Substitute centre back Matheus Felipe was ready on the sidelines. But Athletico chose not to bring him on. The interval was coming. They would wait for half time to make the change. It was surely an error. Former Manchester City midfielder Fernandinho dropped back to fill in at centre back. With the extra man Flamengo were finally able to elaborate some moves on the edge of the area.
On the right flank Everton Ribeiro worked a one-two and crossed behind the line. Fernandinho was lost, and at the far post there was Gabriel "Gabi-gol" Barbosa to turn in left footed and lived up to his nickname. He has scored in all three of Flamengo's recent Libertadores finals.
Athletico made their change at the break, introducing Matheus Felipe. And now their task was almost impossible. Outgunned and a man down, they had to chase the game in the heat against an opponent with the attacking force to blow them away on the counter attack.
By now in a more orthodox 4-4-1, Athletico did what they could. But Everton Ribeiro and De Arrascaeta could now find space between the lines to slip through little passes. Flamengo were by far the more dangerous side, and wasted opportunities on the counter attack. Athletico freshened up their wingers, changed their centre forward and tried hard to get back into the game.
But their only threat came from set pieces. Even attacking throws were treated as major events, with the centre backs trotting forward in the hope that something might fall their way.
They came closest from a free kick, well struck over the wall by Uruguayan substitute David Terans, but safely held by Santos diving to his right. The Flamengo bench were trying to contain their celebrations even before the final whistle, and some of the players and coach Dorival Junior could be forgiven for starting to dream of the prospect of taking on Real Madrid early next year in the final of the Club World Cup. Because after being crowned champions of South America, the next step is the world.