South Korea striker Ji Dong-won netted twice against his former club Borussia Dortmund as the league leaders suffered a shock 2-1 defeat at Augsburg on Friday to stumble again in the Bundesliga title race.
Defending champions Bayern Munich, who are second, will draw level with Dortmund on 54 points if they win at third-placed Borussia Moenchengladbach on Saturday.
The result is a blow for Dortmund as they try to overturn a 3-0 defeat on Tuesday when they host Tottenham Hotspur in the return leg of their Champions League last 16 tie.
Dortmund now have just one win in their last seven matches in all competitions.
They have also dropped nine points in their last five league games.
Both Augsburg goals were a result of mistakes by inexperienced Dortmund defenders Dan-Axel Zagadou, 19, and Achraf Hakimi, 20.
"We practically scored their goals ourselves with two individual mistakes," said Dortmund captain Marco Reus, back after four weeks out with injury.
"We have to stop that. We can't give away possession so easily, we had enough goal chances to win the game."