Rafael Benítez had made a point of warning his players against complacency in the buildup to this contest but, clearly, that message did not entirely sink in. Chelsea secured their passage into a Europa League semi-final at Rubin Kazan's expense but, amid swathes of empty seats in this vast Moscow arena, they were beaten on the night with their fragility causing the interim manager palpitations on the touchline. Manchester City await in the FA Cup at Wembley on Sunday and, on this evidence, they will hardly be quaking in their boots.
Perhaps the slackness that set in after the interval here had been born of the tie feeling settled early, when Chelsea forged 4-1 ahead on aggregate courtesy of Fernando Torres's excellent goal and the Russians' pursuit was left forlorn. Yet there was far too much sloppiness thereafter with David Luiz and Frank Lampard bickering as they departed the pitch at the break, and Rubin players permitted to burst at will through the middle to expose vulnerability in the air on the resumption. The side deemed to be at home, albeit 500 miles from Kazan due to the frozen conditions there, scored three times and clipped the outside of a post, with Premier League opponents left frazzled.
One of those goals, the winner, was a desperately dubious penalty awarded against César Azpilicueta as the substitute Aleksandr Ryazantsev's first contribution was to tumble under no apparent contact. The Spanish full-back could only smile in disbelief, Ryazantsev offering a wink that appeared to confirm guilt, before Bibras Natcho converted. "It was a soft penalty," Lampard said. "Luckily, the game was out of sight." Even so, had José Rondón not guided a close-range header straight at Petr Cech in the time that remained this might still have been a frantic finish.
City will have registered the indecision that had gripped with interest, even if they will confront different personnel on Sunday. Nathan Ake had patrolled central midfield here on his European debut, initially with Ramires at his side and, later, Mikel John Obi.
The teenager was tidy in possession on his second start for the first team, though Rubin sensed their runners might go untracked when sprinting from deep. The seeds had been sown in the moments just before half-time when Chelsea drifted, as if frozen in anticipation of the whistle. Natcho had time and space to chip a pass beyond David Luiz and free Gokdeniz Karadeniz, the Rubin captain poking an attempt at goal that Cech, tumbling to his right, clawed to safety.
That was a fine save, though the concession of the opportunity became a recurring theme. They shipped an equaliser from a short corner routine, Pablo Orbaiz exchanging passes with Natcho before swinging over a cross that another Spaniard, Iván Marcano, thumped into the net.
Victor Moses immediately restored the visitors' advantage on the night with a rat-a-tat buildup involving Lampard and Ramires, but still the visitors dawdled. First the diminutive Karadeniz was permitted to amble away from Ake and nod in Cristian Ansaldi's looped cross, both Paulo Ferreira and David Luiz oblivious to the Turkish player's burst between them, with the ball squeezing through Cech's attempt to save. Then, moments later, Marcano was free again to flick Ansaldi's centre on to the base of the far post.
Rubin's perseverance was admirable, even if Kurban Berdyev's side had been undermined by their own uncharacteristic defensive errors over the two legs. Certainly, they had handed the European champions the early initiative here that should rendered the contest settled.
The crowd were still cooing at John Terry's timely block on a Roman Eremenko shot when the Russians presented Torres with the acres in which to revel. Lampard liberated the Spaniard from the centre circle with a delicious pass, all trademark whip as it was delivered first time beyond defenders who had pushed optimistically up field. Torres galloped away and, having sighted Sergei Ryzhikov indecisive on the edge of his own penalty area, lobbed the goalkeeper with precision and ease.
It was the type of goal he had registered throughout that prosperous period at Liverpool, when Xabi Alonso would send him scurrying beyond back lines apparently oblivious to his searing pace. Back then he might have driven on and rounded the panicked keeper, yet the bounce of the ball on the synthetic surface here made the lob so appealing.
The 29-year-old still sports that mask to protect the nose he fractured against Steaua Bucharest in the previous round, but this competition has offered an opportunity to supplement his industry with goals. It was Torres's fourth goal in three appearances in the competition and leaves Benítez with a decision to make on his selection against City when Demba Ba, cup-tied here and recovering from an ankle problem back at Cobham, should be available again. "Fernando has been working so hard, creating chances for his team-mates and himself," the interim manager said. "We'll check on Demba and see what we have to do for Sunday's team. Afterwards, we will decide."
He will have departed encouraged that the involvement of Juan Mata and Eden Hazard was limited to trots along the touchline, nominally warming up in front of the piled banks of cleared snow, and should be fresh for Sunday. There is confidence to be gained, too, in having secured an 18th semi-final appearance of the Roman Abramovich era at the club. After a season of near-misses, the hope is that this competition may yield silverware. Yet to achieve that, a sense of defensive solidity must return.