Having suffered a comprehensive 4-0 rout in the initial quarterfinal leg, Dortmund, facing a fait accompli on Tuesday, adopted an unbridled attacking posture against Barcelona from the outset, consequently unsettling their visitors.

Notwithstanding his team's commanding four-goal advantage, Barcelona tactician Hansi Flick, possessing considerable prior familiarity with confronting—and vanquishing—Dortmund from his tenure in the Bundesliga, acknowledged that the fervent home support had prefigured an arduous return fixture.

"I harbored a nascent intuition that such a denouement was imminent," Flick confided to the Amazon Prime broadcasting consortium. "It is incumbent upon one today to proffer felicitations to Dortmund, whose performance was exemplary, and equally to my own squad, who have successfully negotiated passage to the semi-final stage."

The normally indomitable Barcelona strike force, featuring the prolific Raphinha and Robert Lewandowski—boasting a combined 23 Champions League goals this season—was conspicuously muted, with Raphinha’s desultory attempt at Gregor Kobel representing their sole discernible threat.

Following a display which emphatically underscored his burgeoning pre-eminence and culminated in his securing pole position in the Champions League scoring charts, coach Niko Kovac hyperbolically lauded Guirassy as Dortmund's "veritable linchpin," adding, "Serhou, as is incumbent upon any spearhead, necessitates the unwavering backing of his confreres, a desideratum amply fulfilled this afternoon."

Borussia Dortmund had arguably forfeited the quarter-final tie in the initial leg, further hampered by the contemporaneous injury-induced unavailability of captain Emre Can and midfielder Carney Chukwuemeka.

For a spell, Guirassy rendered the ostensibly impossible a palpable reality.

Guirassy, having squandered opportunities in the initial fixture and foregone further early prospects in Tuesday's encounter, exhibited no fallibility from the penalty mark in the eleventh minute subsequent to a collision between Barcelona custodian Wojciech Szczesny and Pascal Gross.

Guirassy's header culminated in a two-goal advantage in the forty-ninth minute, prior to Ramy Bensebaini's own goal exacerbating Dortmund's plight—Fermín López's low delivery glancing off Bensebaini's ankle—a compounded setback for Dortmund, given the defender's pivotal role in the attempted resurgence, having assisted Guirassy's preceding goal moments earlier.

Exploiting Ronald Araujo's defensive misjudgment, Guirassy consummated his hat-trick in the 76th minute with a potent close-range strike, thereby assuming the mantle of the Champions League's leading marksman this season with a tally of 13 goals.

This afforded Dortmund a nascent glimmer of hope for engineering what would have been a Champions League reversal of truly historic proportions — one that would have invited comparison with Barcelona's epic denouement against Paris Saint-Germain in 2017 — however, Barcelona resolutely maintained their advantage to secure progression.

Barcelona are slated to contend with either Internazionale Milano or Fußball-Club Bayern München in the penultimate stage, their inaugural foray into the final four of the UEFA Champions League since the 2018-19 campaign. Simultaneously, Paris Saint-Germain clinched their berth in the semi-finals through a 5-4 aggregate victory against Aston Villa, notwithstanding suffering a 3-2 reverse in the return fixture.

Beyond marking Barcelona's initial comprehensive defeat across all fixtures subsequent to their 2-1 reverse against Atletico Madrid on December 21st, this result also represented Barcelona's inaugural Champions League loss since September, constituting merely the seventh setback experienced under Flick's fifty-game tenure; the German tactician having previously secured victory in all seven of his prior encounters against Dortmund whilst at the helm of both Barcelona and Bayern.

A protracted period may elapse before the Champions League anthem resonates through the Dortmund stadium once more.

The prior season's vanquished Champions League finalist languishes in eighth place in the Bundesliga table, six points adrift of the coveted UEFA Champions League berths with merely five fixtures remaining.