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The striker scored either side of half-time before Felix Nmecha completed the scoring in the 81st minute for Dortmund’s first Bundesliga win of the season.

Guirassy’s second goal caught the eye after he completed a neat one-two with Maximilian Beyer before dinking the ball over the on-rushing Frederik Ronnow.

Guirassy has now scored in seven consecutive Bundesliga matches. For Dortmund, only Pierre-Emerick Aubameyang in 2015 (10) and Robert Lewandowski in 2013 (12) have managed to score in more consecutive top-flight matches.

In its first match of the new campaign, Nico Kovac’s side conceded two late goals in its 3-3 draw against St. Pauli after defender Filippo Mane received a red card.

Guirassy believes the players had to put on a strong performance for their home fans after they dropped points in their opening match.

"We had to show a reaction after the bad game at St. Pauli,” Guirassy said.

“That's what we did, we owed it to our fans. That was rehearsed [the build-up for the second goal]. We are working on things like this.”

Kovac, who recently extended his contract with Dortmund, added: “Serhou Guirassy is a very important player for us, who scores the goals even in difficult situations.

“We had a hard time getting into the game. Union bridged it with many long balls. It's about us circulating the ball much faster and having fewer touches.

“After 20 minutes it got better. On the whole, we dominated the game and deserved to win.”