Reus strikes twice as Dortmund trim Bayern's lead
Reus, who won a controversial penalty when the two teams met last season, was given a hostile welcome in the capital.
He silenced the home end with two first-half goals as Dortmund bounced back from last week's loss to Leverkusen to claim their first ever away win at Union.
"Last week was really bad and we wanted to show a reaction today. It's difficult to come here because Union are a really good team," Reus told DAZN.
He batted away questions about the title race.
"We don't need to keep answering those questions, so please stop asking it," he said.
Team mate Mats Hummels was equally unwilling to talk about his side's chances of catching runaway leaders Bayern, pointing instead to the fact that Dortmund now have a 12-point cushion in the race for the Champions League.
"I am just happy that we've opened up a huge lead on fifth place," he said.
After leaders Bayern slumped to a shock loss at minnows Bochum on Saturday, the clash in the capital was a must-win for second-placed Dortmund.
Dortmund made a nervy start, but took the lead on 18 minutes, Reus drilling a low shot home from close range amid a flurry of legs in the box.
Reus doubled the lead on the counter-attack shortly afterwards after Dan-Axel Zagadou caught the Union defence napping apart with a devastating diagonal ball upfield.
Union's valiant second-half resistance was ended on 71 minutes when Raphael Guerreiro toe-poked in a third goal from close range.
Tempers flared in the final 15 minutes as the home side had a goal ruled out by VAR for a foul on Manuel Akanji.
But Dortmund held on to keep their first clean sheet in seven games.
Marco Rose's side now turn their attention to Thursday's Europa League clash with Scottish champions Rangers.
"A lot of our players have never played in the Europa League, and Rangers are a really good team. It's important that we enjoy this win and let out the frustration from last week, and then we can focus on Rangers from Tuesday," said Reus.
Hoffenheim end losing streak
Defeat dropped Union to seventh, while Hoffenheim leapfrogged them into fifth with a 2-0 win at home to Arminia Bielefeld in Sunday's later game.
Hoffenheim took the lead on 22 minutes when Benjamin Huebner's header clattered off the bar, into the back of goalkeeper Stefan Ortega and in.
Georginio Rutter smashed the ball into the top corner from close range just after half-time to double the lead, ending a run of four defeats in a row for Sebastian Hoeness' side.