This ended up a timely reminder of the resilience, and ruthlessness, Chelsea can still command on this stage. A tie that might have veered away from them, particularly while they laboured to contain Slavia Prague’s urgent intent, was claimed late on by a marauding full-back, breaking late into the box to meet an inviting centre. That, at the very least, was an indication of this team’s underlying ambition.

Marcos Alonso has not been himself since the onset of winter so it was reassuring to see him galloping untracked between defenders again, even in the dregs of the game, to meet Willian’s cross before planting an emphatic winning goal beyond Ondrej Kolar.

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Chelsea will boast another dimension to their attacking play if they can coax a consistent threat from their left-back, so integral had he been to their encouraging progress under Maurizio Sarri’s stewardship at the start of the campaign.

The finish served to inflict Slavia’s first home defeat since August and probably one their tenacious display had not warranted given the extent to which they had unnerved the visitors. Yet, in adversity, Sarri spied promise for the decisive run-in ahead. “In the past, when we have been in trouble, we’d concede immediately a goal,” he said. “Now we are able to stay in trouble, to fight, to suffer without conceding anything. And so that’s a step forward for us.

“This was a very difficult game, so I’m really happy with the result and also with the performance because, in this moment, we are able to suffer.” They may need to weather a wilder storm at Anfield on Sunday.

It had taken Kepa Arrizabalaga, blocking from Ibrahim Benjamin Traoré and Jan Boril, to preserve the clean sheet while Chelsea, unsettled by their hosts’ energy, personified by Peter Olayinka’s constant running, laboured to retain possession or make in-roads of their own.

This quarter-final represented the sternest test of a kind Europa League campaign and for all that Willian had whipped a fine shot on to the crossbar midway through the opening period Chelsea had mustered little threat of their own until Alonso’s unexpected intervention.

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Sarri would fling on Eden Hazard, N’Golo Kanté and Ruben Loftus-Cheek from the bench in a bid to impose some control, but the goal was wholly against the run of play.

Slavia probably deserved better, for all that their efforts did not yield enough clear-cut opportunities. A team who had managed four goals while emerging out of the group before Christmas – they had since scored four at Genk and at home to Sevilla in the knockout phase – reverted to type, with excellent approach play running aground on Chelsea’s stubborn defence. Antonio Rüdiger, bellowing instruction at team-mates in a bid to raise standards, repelled everything flung at him. César Azpilicueta, dragged out of position at times, was as obstinate and unflappable as ever.

“They were aggressive, but we were able to face them well,” said Sarri.

“The second leg will be just as difficult because these opponents are just as dangerous away from home, as they showed in Genk.”

This tie has not yet been properly claimed, but the advantage is undoubtedly with Chelsea.