Struggling Augsburg blew the Bundesliga title race wide open as leaders Borussia Dortmund slid to only their second league defeat of the season at the WWK Arena.
Ji Dong-won returned to haunt his former club as he scored in either half to render Paco Alcacer's effort a mere consolation for Dortmund, who missed out on the opportunity to pull six points clear at the top of the standings following a 2-1 loss.
Instead, rivals Bayern Munich can move level on points with Dortmund if they overcome third-placed Borussia Monchengladbach at teatime on Saturday.
The win for Augsburg significantly enhances their survival prospects as they moved five points clear of the relegation zone.
Augsburg were indebted to their South Korea striker Ji, a former Sunderland striker who made five appearances for Dortmund's reserves five years ago.
The 27-year-old shook off the challenges of two defenders to open the scoring at the second attempt from close range in the 24th minute.
Dortmund, who included captain Marco Reus in their line-up following a month on the sidelines because of injury, struggled for fluency but Thomas Delaney and Jacob Bruun Larsen went close to levelling.
Augsburg though were resolute in defence and Ji doubled his tally in the 67th minute with an excellent chip over Dortmund goalkeeper Roman Burki.
Alcacer, introduced shortly before Augsburg's second goal, set up a grandstand finale when he scored his 13th league goal of the campaign nine minutes from time but Dortmund were unable to find an equaliser.
