Three and a half weeks ago, the blue-and-white-shirted hordes faced a long, sad drive home after crossing the border to Brazil to watch Argentina lose in the final of the World Cup.
Now there has been another Buenos Aires invasion of a neighbouring country -- some 8,500 San Lorenzo fans travelled to Paraguay to cheer on their team against Nacional in the first leg of the final of the Copa Libertadores, South America's Champions League.
The San Lorenzo fans can return in better spirits than those who went to Brazil for the World Cup.
Their team are still favourites to win the title for the first time after a 1-1 draw in Asuncion. But their mood would surely be better but for Nacional's last gasp equaliser, denying San Lorenzo what would have been their first away win of the campaign -- a victory they would have deserved on the balance of play.
Neither side was at full strength.
Nacional's holding midfielder Marcos Riveros was suspended. But more significant was the failure to recover from injury of centre forward Brian Montenegro.
Without him, Nacional lacked the attacking pace to expose San Lorenzo's reserve centre-back, Fabricio Fontanini.
The home side never established a platform in the game, and were unable to get their hard working pair of midfielders, Silvio Torrales and Derlis Orue, up in support of the strikers.
Instead, San Lorenzo controlled the middle of the pitch with Nestor Ortigoza and Juan Mercier -- the outstanding player in this year's competition -- dominating the action.
San Lorenzo are an experienced side and are well coached by Edgardo Bauza. As the game wore on their little partnerships began to click and impose themselves; Ortigoza and Mercier in the centre, ahead of them striker Mauro Matos and playmaker Leandro Romagnoli.
On the left, the clever Ignacio Piatti combined well with full-back Emmanuel Mas. On the other flank, quick Hector Villalba worked well with attacking right-back Julio Buffarini.
And nearly 20 minutes into the second half, all the common sense collective play brought them a goal; Mercier spread the play to the right, Buffarini cut infield and worked a one-two with Romagnoli before sliding back out for Villalba to hit the danger zone with a cross.
Matos did well to get in front of his marker and direct a volley across the goalkeeper and inside the far post.
It was a goal that summed up the good things that San Lorenzo had done in the game, and seemed set to give them a first leg win -- and force Nacional to win their first away game of the campaign in next week's return match.
It is always easy in hindsight, though perhaps coach Bauza will conclude that his men did not do enough to retain controlled possession and maybe kill off the tie in the first leg.
But it was hard to see where a Nacional goal would come from. Home coach Gustavo Morinigo made some attacking substitutions, and was rewarded when two of the players he introduced were involved in the equaliser.
San Lorenzo cleared their lines and were too slow to push upfield, allowing Hugo Lusardi, one of those substitutes, the time and space to send in a long, diagonal left footed ball from deep.
Fredy Bareiro nodded on, and another late replacement, Julio Santa Cruz, stretched in front of Fontanini to send his shot into the roof of the net.
It was an emotional moment for the scorer, in the same Defensores del Chaco stadium where his older brother Roque has notched so many great goals, and it sets up next week's second leg -- when, whatever happens, a new name will be written on the Copa Libertadores trophy.
Tim Vickery is an English journalist who has been based in Brazil for the past 20 years. He is the South American football correspondent for the BBC Sport.