Kagawa scored his ninth goal of the season in the eighth minute as Dortmund, who also hit the woodwork twice in the first half, extended their unbeaten Bundesliga run to a club record 20 games.

The titleholders have 59 points from 26 games, eight clear of Bayern Munich, who were away to Hertha Berlin later on Saturday (1730), and Borussia Moenchengladbach, who snatched a 2-1 win at Bayer Leverkusen with Igor de Camargo's 88th minute goal.

Six-times champions Hamburg, 14th with 27 points, are only two ahead of Freiburg in the relegation playoff spot after a crushing home defeat.

Johannes Flum and Daniel Caligiuri gave the visitors a 2-0 halftime lead and Cedrick Makiadi added a third before Ivo Ilicevic grabbed a consolation for Hamburg. There was more bad news for Hamburg as Augsburg, another relegation-threatened team, beat Mainz 05 2-1.