Barcelona suffered their first home Primera Liga defeat since April 2012 on Saturday night as Valencia registered a shock 3-2 win at at Camp Nou.
The loss has seen Barça leapfrogged by both Atlético and Real Madrid in the table, as their aura in the post-Pep Guardiola era continues to diminish.
Alexis Sanchez gave the champions the lead before Dani Parejo slotted in and Pablo Piatti nodded in for the visitors. Lionel Messi made it 2-2 from the penalty spot but Paco Alcacer swept home to give Valencia a famous victory, as Jordi Alba was sent off.
Jordi Alba is dismissed during Barcelona's defeat to Valencia
Barça's second league defeat this season allowed Atléti to capitalise, and Diego Simeone's team have sugred to the top of the table for the first time in 18 years following their 4-0 thrashing of Real Sociedad on Sunday.
Simeone has done great work at the Vicente Calderón, having guided the club to the 2012 Europa League win before clinching the Copa del Rey over Real Madrid at the Santiago Bernabéu last year.
Their triumph across town was their first win against Real since 1999, and they defeated their city rivals again in the league back in September.
Despite losing Radamel Falcao in the summer, Diego Costa has performed superbly and trails only Cristiano Ronaldo in the goalscoring charts.
Ronaldo was controversially sent off as Real drew 1-1 at Athletic Bilbao following Atlético's win at the weekend.