Atletico Madrid may have to fight to the end to finish in La Liga's top four but the success of their season will depend on holding on against Liverpool in the Champions League on Wednesday.
At the end of a breathless 2-2 draw with Sevilla on Saturday, Atletico sat fifth in the table, behind both their opponents and Getafe, with Real Sociedad able to overtake them too if they win their game in hand on Tuesday.
It means the possibility of Atletico missing out on Champions League qualification for the first time in seven years is very real and will come into even sharper focus next weekend should they slip up at a resurgent Athletic Bilbao.
By then, Diego Simeone's side will either be lamenting a second consecutive exit in Europe before the quarter-finals or flying high, after pulling off a momentous upset by knocking out the reigning champions at Anfield.
Few gave Atletico even an outside chance ahead of their throwback display at the Wanda Metropolitano last month, when guts, determination and diligence gave them a shock 1-0 lead to preserve in the second leg.
Asked to describe his team's performance, Simeone said: "The word that comes to mind is calm, that tranquility that you feel when you work as a team and know you can do anything."
And while after the match Liverpool boss Jurgen Klopp bristled over Atletico's approach and Andy Robertson complained about the extent of their celebrations, the sense was still of an aberration, a momentary error that Liverpool would soon correct on their own patch.
"We don't think it will be easy but as long as we can get 11 players in Liverpool shirts we will try everything we have," said Klopp. "And for all the Liverpool fans that can get a ticket for the game, welcome to Anfield."
"They've celebrated as if they've won the tie after the game, so let's see," said Robertson.
For Atletico, the challenge is to prove the doubters wrong again, not least because that is the scenario they and Simeone revel in most.
Liverpool might be encouraged by their opponents' continuing problems since, with a win and two draws - one of them against La Liga's bottom club Espanyol - hardly pointing to a team inspired to leave mediocrity behind.
Yet Liverpool have been suddenly wobbling too.
On the other hand, while their demolition of the Premier League makes the Champions League something of a bonus, Atletico's failure in La Liga means progress in Europe is their remaining hope.
Some in Spain have suggested Simeone should keep one eye on the game against Athletic Bilbao and be cautious with the fitness of recently-returning players such as Joao Felix, Diego Costa, Renan Lodi and Kieran Trippier.
"Now comes the Liverpool clash," wrote Madrid newspaper AS on Monday. "But it's not the most important for the club. The decisive one is in Bilbao because for there to be many more games like Anfield, Atletico has to win games like the one at San Mames."
But Simeone's conservative style masks an intrepid belief and his decisions in the second half against Sevilla, including taking off Joao Felix despite criticism from the fans, suggest he is putting Liverpool front and centre.
Simeone might even think victory could open up an opportunity. With almost all of Europe's heavyweights seemingly out of sorts, his team would certainly have little to fear in a draw for the quarter-finals.
The odds are not in favour of Atletico holding out for another 90 minutes against the world's most fearsome attack or discovering a precision in front of goal that has been so painfully missing for the best part of seven months. But if they pull it off, their season could spring into life.