Borussia Dortmund dropped to 10 points off leaders Bayern Munich after losing to second-placed Bayer Leverkusen.
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Leverkusen moved up to four points behind Bayern, who beat Werder Bremen 7-0, thanks to Son Heung-Min's goal.
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The South Korea international rounded Roman Weidenfeller to fire home after 18 minutes. Leverkusen had Emir Spahic sent off in a disputed decision for a tackle and Dortmund's Sokratis Papastathopolous followed for a second booking.
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Dortmund's season is in danger of being all but over by Christmas, with a crucial game against Marseille on Wednesday, where a defeat could eliminate last season's finalists from the Champions League.
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But Sami Hyypia's Leverkusen are still in title contention after a deserved victory in Dortmund.
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Borussia Dortmund striker Robert Lewandowski had a shot saved by keeper Bernd Leno before Son scored the opener.
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The summer signing from Hamburg was let in by a defensive mistake from Manuel Friedrich, the 34-year-old who left Leverkusen in the summer and joined Dortmund following an injury crisis last month.
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Son was denied a second by a fantastic save from Weidenfeller a few minutes later and Jens Hegeler just chipped over as the visitors dominated the first half.
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Spahic was sent off with 10 minutes left to give Dortmund hope after the defender went in two- footed on Lewandowski, although without raising his studs.
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Dortmund had renewed hope and Lewandowski forced a save from Leno before Papastathopolous threw the ball away in anger and was sent off for a second yellow card. Pierre-Emerick Aubameyang fired a free-kick over and substitute Marco Reus forced a fantastic save from Leno but the hosts could not find a crucial equaliser.
Injuries worries for Borussia Dortmund.
Sad times for Klopp and Dortmund fans.