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Jurgen Klopp's team came back strongly in the game thereafter and scored three goals in the final five minutes of the first half.
Sahin himself etched his name on the scoresheet three minutes later, firing in with his left foot from the left side of the Freiburg box.
Lewandowski virtually ended the game as a contest in first-half injury time when he found himself with a loose ball 10 yards out and made no mistake with a simple shot.
The Turkish midfielder squandered a good chance to score his second goal of the afternoon in the 69th minute when he failed to find the target from 18 yards from a good position, but made no mistake three minutes later from close range.
Leonardo Bittencourt made it 5-1 to Dortmund with 12 minutes of normal time left on the clock, turning in Lewandowski's delivery across from the face of the Freiburg goal with ease.