Guirassy guides Dortmund past Atalanta in Champions League play-offs
Despite a late arrival delaying kick-off and just one recognised centre-back, Dortmund controlled the match from start to finish.
The in-form Guirassy headed Dortmund in front early and then set up Maximilian Beier's goal late in the first half to give the Germans the advantage ahead of next week's return leg in Italy.
"It was a perfect start but a weird evening," Guirassy told Amazon Prime.
"We were in traffic for an hour and 15 minutes and only had 10 minutes to get ready and get on the pitch after we arrived, so it was nice of course to get the header after three minutes."
Dortmund midfielder Julian Brandt told reporters a 2-0 lead was "still dangerous" ahead of the second leg in Bergamo but praised his side's focus despite the pre-match setbacks.
"A lot is working for us now. Even in difficult situations, we're managing it.
"We were able to convert the stress into energy when we started. It sharpened us up a bit today -- but it shouldn't become the norm," Brandt said of the late arrival.
Atalanta arrived on time but were out on their feet in defence and disjointed in attack, missing injured midfielder Charles De Ketelaere's attacking spark.
The winners of the tie will face either Arsenal or Bayern Munich in the last 16, the teams who finished first and second respectively in the league phase.
Despite a run of six straight Bundesliga victories, Dortmund were hit with a defensive crisis, with Nico Schlotterbeck, Niklas Suele, Emre Can and Filippo Mane all out injured.
Coach Niko Kovac handed 18-year-old Luca Reggiani a first senior start alongside Germany defender Waldemar Anton, Dortmund's last centre-back standing.
The hosts' preparation was further hampered by a delayed arrival at the stadium, with kick-off pushed back by 15 minutes.
Bursting back into form after a long dry spell with five goals in three games, Guirassy wasted no time continuing his streak.
With three minutes gone, the Guinean nodded in a Julian Ryerson cross to give Dortmund the lead on three minutes.
Dortmund doubled their lead just before the break when Guirassy delivered an expert pass across the face of goal for Beier to tap home.
The hosts were content to control proceedings with their two-goal lead, with few chances for either side in the second half.