SATV 16 April, Kathmandu: After being swept aside 4-0 in the first leg of the quarter-final, Dortmund had nothing to lose and attacked Barcelona from the start.
Guirassy had missed chances in the first leg and spurned a couple more early in Tuesday's game but made no mistake from the penalty spot in the 11th minute after Barcelona goalkeeper Wojciech Szczesny collided with Pascal Gross.
Guirassy's header made it 2-0 in the 49th minute before an own goal by Ramy Bensebaini made Dortmund's task even harder.
Guirassy seized on Ronald Araujo's defensive error to complete his hat-trick with a powerful close-range shot in the 76th, in the process becoming the Champions League's top scorer this season on 13 goals.
It gave Dortmund fresh hope of completing what would have been one of the all-time great Champions League comebacks - one to rival Barcelona's against Paris Saint-Germain in 2017 - but Barcelona held on to progress.
Barcelona, who suffered their first loss in all competitions since December, will face either Inter Milan or Bayern Munich in the semi-finals.
It's the first time Barca, who suffered a first Champions League defeat since September, have reached the last four of the Champions League since the 2018/19 season.