After just one win in six, Die Schwarzgelben could hardly afford to drop more points, especially against a Union Berlin side that has lost each of the five previous meetings hosted by BVB in the Bundesliga.
Despite Union creating the first opening of the game when Danilho Doekhi headed over from close range inside five minutes, Dortmund enjoyed a strong opening 45 minutes.
They broke the deadlock around the midway point of the half, albeit thanks to a huge slice of luck as Julian Ryerson’s low drive that was going wide deflected off Diego Leite and looped over Frederik Ronnow before nestling in the back of the net.
Shortly after, Pascal Gross struck the crossbar from close range as the home team turned the screw. Benedict Hollerbach did test Gregor Kobel, but Dortmund ultimately went into the break two to the good.
Gross was involved, curling a perfect cross onto the head of Guirassy, who could hardly miss as he netted his 20th goal of the season in all competitions.
After the break, Dortmund were in almost total control, and almost extended their advantage further when Guirassy fired narrowly over the crossbar.
Maximilian Beier forced Ronnow into an admittedly comfortable save before missing the target totally when unmarked at the back post, while Gross worked the Union goalkeeper in between those two efforts.
Karim Adeyemi then struck the foot of the post prior to Guirassy’s second goal of the game, hammering into the roof of the net from six yards to effectively kill off the contest.
The former VfB Stuttgart man then netted a third when he tapped in effectively from the goalline after Beier’s pass.
In response, Union offered very little, which is hardly surprising for the second-lowest goalscorers in the Bundesliga this season.
Their evening was made worse, as Guirassy found the back of the net once more, this time with his head from a Gross cross.
And the scoring was finally completed by Beier, again with Gross providing the assist.
This defeat leaves Union 10 points clear of the automatic relegation places, and nine above the relegation play-off spot. Dortmund, meanwhile, end a run of two consecutive losses, and perhaps more importantly, keep a second clean sheet in a row.
Flashscore Man of the Match: Serhou Guirassy (Borussia Dortmund)