Diego Simeone's had to come from behind to gain the point that saw them crowned champions of Spain on a dramatic final day of the season at Camp Nou.
Their 1-1 home draw against Malaga last week forced the title race to a thrilling conclusion, with Simeone's side set to claim the league with a win or a draw - but Barcelona still able to defend their crown with victory.
Alexis' strike, which arrowed into the top-right corner from a highly unlikely angle, came after Lionel Messi chested down Cesc Fabregas' lobbed ball.
And they were ultimately denied by Atleti's typically stubborn defence as the visitors ended their Liga drought.
The manner of Atletico's win – and their season – saw them applauded by the Barcelona fans after the final whistle had blown.
A Gareth Bale strike and late double from Alvaro Morata proved to be enough for hosts Real in their final game of the domestic season, despite Pizzi's consolation for Espanyol, but the victory was marred by setbacks to Cristiano Ronaldo and Karim Benzema.
That puts his involvement in the final of Europe's premier club competition under question, and Real could also be without Benzema after he came off midway through the second half with an apparent groin complaint.
Real dominated the match and ensured victory with a late flurry from Morata – who replaced Benzema off the bench – but the focus will firmly be on the prospects of their injured duo.
