After 31 minutes of sound and fury at Anfield last week, Liverpool were 3-0 up and threatening more punishment against a bedraggled Manchester City.
After 31 minutes of sound and fury at the Etihad Stadium on Tuesday, Liverpool were 1-0 down, had failed to register a shot on goal and Loris Karius seemed to be pondering the meaning of life before every goal-kick.
Jurgen Klopp warned his players to expect a thunderstorm from a City side wounded by a painful week and it arrived following the first whistle from Antonio Mateu Lahoz. The Spanish official would blow far more contentiously as an absorbing evening gripped the senses a little tighter.
Of all the elements in this sky blue storm, Raheem Sterling was the howling wind as he turned in a first-half display of relentless catharsis against his former employers.
Sterling has been through the mill plenty of times against Liverpool since joining City in 2015. He missed a sitter in the 2016 EFL Cup final, found himself parked unforgivingly on his backside by Jon Flanagan in a league game a few days later and was substituted to howls of derision as Pep Guardiola's Premier League champions elect were ransacked on Merseyside in January.
The 23-year-old decided it was time to dish out some hostility of his own, jackhammering after Virgil van Dijk to force a wayward clearance. He dumped the towering Dutch defender on the turf for good measure.
Lahoz waved "play on", Fernandinho found Sterling and the England winger showed the calmness under pressure that deserted him during Saturday's Manchester derby defeat. Gabriel Jesus could not miss.
The storm raged and Liverpool battened down the hatches as best they could. Sterling went to ground twice in the area, with a nervous Andrew Robertson spared; Bernardo Silva cracked one against the post and Leroy Sane had a goal incorrectly ruled out for offside.
The latter decision in particular seemed to tip Guardiola over the edge. Not enjoying the benefit of a language barrier with this referee, some choice words at the interval left the City manager in the stands for the second half.
Without the Catalan's cajoling presence, City's suffocating intensity subsided. Liverpool and Klopp breathed. A storm always blows itself out eventually.
The clouds parted and Mohamed Salah spied the semi-finals. There they were, as Ederson threw himself at Sadio Mane's feet and the ball spun free. Salah's touch was immaculate; his chipped finish even better.
It lent further weight to the Egyptian superstar's claims for the end-of-season awards, his 39th goal of a remarkable campaign.
But, at the risk of taking the phenomenal Salah, Mane and Roberto Firmino combination for granted, we know what they do to tired and emotional defenders – just ask Nicolas Otamendi after Firmino picked his pocket to seal a 2-1 win on the night 13 minutes from time.
The questions Liverpool answered on Tuesday were all asked in the white heat of their own penalty area. This was not a smooth defensive performance that the AC Milan of Franco Baresi and Paolo Maldini would have cheered, but it got the job done.
Van Dijk shrugged off that bruising start to again helm his back four with authority. Trent Alexander-Arnold dealt manfully with Sane once more, never looking like losing his head after a deserved first-half booking.
More than any of Klopp's victories over ex-Bayern boss Guardiola - that's three this season alone in a campaign where City have only lost seven times in total - this was one hewn from the no-nonsense, every-last-drop-of-sweat school he led as a senior player and a fledgling coach at Mainz. Run harder, run longer and prevail.
His team are showing a hard edge at just the right time. With favourites Barcelona stunningly out, ageing Bayern Munich and Real Madrid sides would find plenty to detest about playing Klopp's Liverpool. Maybe, just maybe, their red lightning can strike at the heart of European football.
