Sporting scored a dramatic late winner to secure their place in the top eight of the Champions League and progress directly to the last 16.
The Portuguese side trailed at the break at San Mames stadium having given themselves a strong chance of automatic progression by stunning reigning champions Paris St-Germain last week.
But early changes in the second half gave them the renewed energy they needed and Alisson Santos drilled into the corner in the 94th minute to cement their place in the knockout stage.
Athletic started the better but Oihan Sancet's slotted finish in the third minute was cancelled out just nine minutes later by Ousmane Diomande's powerful header from Maximiliano Araujo's corner.
Gorka Guruzeta restored the hosts' lead, nipping the ball away from Matheus Reis then smashing an effort off the post before lashing in the rebound to give them the half-time lead and hopes of reaching the play-offs.
But Sporting's second-half changes paid off as the visitors found more space in behind the Athletic defence and Francisco Trincao neatly finished past Unai Simon to restore parity.
Athletic were unable to rediscover the energy and intensity of the first half as Sporting dictated play, while the Portuguese side also had a penalty decision overturned.
Referee Felix Zwayer awarded a spot-kick when he adjudged Adama Boiro to have brought down Geny Catamo but changed his decision after being sent by the video assistant referee (VAR) to the monitor.
A draw would have left Sporting with a two-legged play-off tie to reach the last 16, but instead Santos picked up the loose ball after keeper Simon denied Luis Suarez to seal a dramatic win.
Meanwhile, Athletic are eliminated from the competition having lost four matches.
Watch highlights of every Champions League game from 22:00 on Wednesday on BBC iPlayer and the BBC Sport website and app.
There will also be a Champions League Match of the Day on BBC One on Wednesday, from 22:40 to 00:00.
