Athletic made a flying start in front of their own supporters, as Hugo Rincon’s delivery picked out Alex Berenguer, who squirmed a golden opportunity wide of Alex Remiro’s goal from inside the area.
Los Leones came into the encounter unbeaten in their previous five home league meetings with Sociedad (W4, D1), and that run looked set to continue in the 14th minute when they broke the deadlock.
Nico Williams’ defence-splitting ball over the top picked out his brother Inaki, who calmly lifted the ball over the onrushing Remiro despite Mamadou Sarr’s best efforts to clear.
Edin Terzic’s men nearly added a second moments later as I. Williams played an inviting ball into the box, but Berenguer failed to connect.
However, despite Athletic’s onslaught, Sociedad were back level on the half-hour mark. Luka Sucic delivered a bolt from the blue, firing low beyond the outstretched Unai Simon and leaving the Yashin Trophy winner helpless.
The visitors immediately went about trying to restore their advantage, with Ander Barrenetxea volleying at Simon before the goalkeeper denied Mikel Oyarzabal.
In first-half added time, they finally got their noses in front once more when Berenguer stepped up and somehow squeezed a penalty underneath the body of Remiro after N. Williams was brought down by Jon Aramburu in the box.
Terzic’s side started the second half where they left off, as I. Williams’ acrobatic effort drifted wide of Remiro’s post. Nevertheless, Sociedad subsequently went close at the other end, but Sucic blazed over from the edge of the area.
The teams continued to go toe-to-toe, and I. Williams was again involved when his chip was thwarted by Remiro.
The slender advantage maintained the visitors’ hopes of salvaging something, and Simon needed to be alert to push away Carlos Soler’s attempt with around 20 minutes remaining. Soler then had another opportunity, only to blaze over after being picked out by Oskar Oskarsson.
La Real rallied in the final few moments as they went in search of a point, but Athletic ultimately ran out worthy winners.
The victory sees the hosts move four points behind Sociedad in fifth with a game in hand, while extending their unbeaten run at San Mames to five matches (W3, D2).
Meanwhile, Pellegrino Matarazzo’s men missed the chance to strengthen their position in the European places, with their six-match unbeaten run in all competitions coming to an end.
Athletic will look to build on their impressive win when they travel to Osasuna next, while Sociedad welcome Olympique Lyonnais to San Sebastian in the UEFA Europa League on Thursday.
Flashscore Man of the Match: Nico Williams (Athletic Club)