Marco Reus' first-half goal set Dortmund on their way to a win on Wednesday that keeps them four points behind Eintracht Frankfurt in the last qualification place for the Champions League with four games of the season remaining.

But Union left the result in doubt until the 88th minute, when Julian Brandt set up Guerreiro to score on a counter-attack.

Dortmund's breakthrough in the 25th minute had more than a touch of fortune.

Union goalkeeper Andrea Luthe was penalised for bringing down Reus, though replays suggested it was a dive but the penalty was still given after a VAR check.

Luthe then atoned by saving Erling Haaland's penalty, and his effort from the rebound, before defender Robin Knoch's attempted clearance off the line went in off the inrushing Reus.

Haaland endured a frustrating game, foiled by Luthe again in the 86th, before he hit the side netting in injury time.

Third-place Wolfsburg stayed on track for Champions League qualification with a 3-1 win at Stuttgart which keeps them five points ahead of Dortmund.

Mainz won 1-0 at relegation rivals Werder Bremen to leapfrog the home side and move five points clear of Hertha Berlin in the relegation zone.

Hoffenheim came from two goals down at halftime to beat Borussia Monchengladbach 3-2.

Australian Associated Press