This was a clash between two Spanish footballing powerhouses who had fallen on tough times and looked to the Copa del Rey as potential salvation after disappointing LaLiga campaigns to date.
Luis Rioja had an early chance in a promising start for the hosts, but it was the 24-time winners who struck first in fortuitous fashion, as Umar Sadiq headed Robert Navarro’s free-kick into his own net after an apparent miscommunication with Dimitri Foulquier.
Defensive mishaps were to be a theme of this first half, and Sadiq would redeem himself for his earlier own goal when Alex Padilla spilt Rioja’s strike directly into the path of the Nigerian to slot home the loose ball.
Athletic pushed forward in a first half which ebbed and flowed both ways heading towards the break, as Mikel Jauregizar’s diving header went wide from Navarro’s searching cross.
Extra time was looking increasingly inevitable after the break, with both sides struggling to produce that moment of quality in search of a second goal.
Nico Serrano was the Lions’ key protagonist going forward, firing wide with two speculative attempts that failed to trouble Stole Dimitrievski.
Foulquier came closer down the other end for Los Che when his close-range strike cannoned off the post at the end of a well-worked move.
However, Ernesto Valverde’s side were awarded a penalty following a lengthy VAR review which ultimately penalised Cesar Tarrega for handball inside the box. Jauregizar didn’t accept the gift from the spot, with his poor attempt from 12 yards parried to safety by Dimitrievski, giving a huge lift to the majority of fans inside a packed Mestalla.
Just when extra time seemed like a formality, the Williams brothers then took centre stage to propel Valverde’s team into the last four.
It continued Athletic’s proud history in this competition, and extended their winning streak to eight away Copa del Rey ties in a row.
Meanwhile, Valencia’s sorry campaign reached another low point after their six-match unbeaten home run across all competitions came to an end.
Flashscore Man of the Match: Nico Serrano (Athletic Club)