Athletic arrived in Madrid riding an eight-game undefeated streak, but Los Franjirrojos paid little heed to the visitors’ form.
The hosts seized an early lead inside the opening quarter-hour when Randy Nteka pounced on a disastrous back pass by Aitor Paredes, coolly burying a lobbed effort over an onrushing Julen Agirrezabala for his side to open the scoring for the first time in three league outings.
This also snapped Athletic’s streak of three consecutive H2H clean sheets at Vallecas, denying them a chance to equal their record of four away shutouts against a single opponent in the top flight, last achieved in 1982.
The Basques sought an immediate response, with Yuri Berchiche and Nico Williams both narrowly missing from long range, as Augusto Batalla proved a reliable guardian between the sticks.
The Argentine produced a vital save to deny Alex Berenguer’s thunderous strike from distance, ensuring the hosts carried their slender lead into the interval.
Unbeaten since early October, Ernesto Valverde sought to inject energy into his side with a triple substitution. The change quickly bore fruit, as Oihan Sancet, one of the fresh entrants, made an instant impact by meeting Nico Williams’ precise cross at the far post and unleashing a brilliant volley into the bottom left corner, levelling the score in the 66th minute.
Red-hot Sancet then completed his brace in spectacular fashion, unleashing an unstoppable thunderbolt from outside the area into the far corner, sealing a remarkable turnaround with 12 minutes remaining.
That strike sealed the deal, as Athletic extended their streak of conceding no more than once in their last nine matches thanks to their defensive prowess, against a Rayo side who have failed to net more than once in their last five league games.
Under Inigo Perez, Rayo have now suffered their third consecutive LaLiga defeat, marking their worst 14-game start to a top-flight campaign since 2018/19 - a season that unfortunately ended in relegation.
Meanwhile, the Lions remain unbeaten in the top flight since falling narrowly to Girona in early October. Athletic climbed into the top four with the victory - a result that also marked their 11th triumph in 14 top-flight H2H encounters against the side from the Spanish capital.
Flashscore Man of the Match: Oihan Sancet (Athletic Club)