Union scored three times in the second half to beat relegation-threatened Hertha 4-1 in the Berlin derby on Saturday and boost their hopes of a European spot while claiming bragging rights in the German capital.
Goals from Grischa Proemel, Sheraldo Becker and Sven Michel made it 4-1 after Genki Haraguchi had put Union in front in the first half and Hertha had briefly levelled with an own goal from Timo Baumgartl after the break.
Union are in seventh place on 44 points, one behind fifth-placed Freiburg. The team in fifth will qualify for the Europa League while the sixth-placed finisher will compete in next season's Europa Conference League.
Hertha are anchored in 17th place on 26, as many as 16th Arminia Bielefeld who sit in the relegation playoff spot, with five games remaining.
Union, who have now beaten their city rivals twice in the league this season and once in the German Cup, had the upper hand at the start, boxing them in their own half and taking the lead through Haraguchi's 31st-minute header.
With the visiting fans lighting dozens of flares in the stands, the hosts responded after the break and were rewarded for a strong start to the second half when defender Timo Baumgartl turned a Hertha cutback into his own net in the 49th.
Their joy, however, lasted four minutes with Proemel placing a superb header past keeper Marcel Lotka to put Union back in front.
They got their third goal with a quick break and Becker picked his spot beautifully to beat Lotka with his shot before Michel completed a memorable win.