Romelu Lukaku had been the subject of suffocating attention from a well-organised and robust Zenit St Petersburgcentral defensive three. Everywhere he turned, it felt as though a door was slammed in his face and, as this tie entered its final stretch, the visitors had to be starting to think they had him and Chelsea under control.

But it takes only one second for Lukaku to break free. It takes him only one move or moment to mete out the harshest punishment. It is the reason why Chelsea dug so deep to re-sign him over the summer and it was the reason why they were able to make a winning start to their Champions League defence.

When the ball was worked from the inside left to the inside right for César Azpilicueta, the Zenit defender Dmitri Chistyakov seemed to have Lukaku within his grasp inside the six-yard box. But then Azpilicueta stood up a lovely cross to the back post and Chistyakov did not. Lukaku had spent the previous 68 minutes waiting, working, lurking. Now he had the big chance and he rose to head it down and in with trademark ruthlessness.

Thomas Tuchel had commented on Monday it was good that Lukaku had hit the ground running on his return because strikers do not like to wait for their goals. Lukaku is simply on fire. He now has four goals in four games and, despite the small sample size, it is easy to think that Chelsea have found the missing piece in the puzzle.

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Lukaku’s header did not mean that it was over because Zenit had a huge opportunity to equalise when Sander Azmoun crossed for the substitute, Artem Dzyuba, only for him to stretch and slice wide with the goal gaping. The concession of the chance was surprising given how miserly Chelsea had been at the back. But they duly closed out the result to give Tuchel the victory he had called for. It is a long way to the final, which will be played at Zenit’s Gazprom Arena but, with Lukaku on board, Chelsea can dream of another triumph.

Romelu Lukaku climbs highest to head home the only goal of the match. Photograph: Chloe Knott – Danehouse/Getty Images

“The performance from Romelu was not easy,” Tuchel said. “We didn’t create many chances for him but he’s the type of guy who doesn’t lose confidence. That is why he is a world-class striker. It gives the team a lot of belief that one chance, or a half-chance, might be enough. It’s more than talent he brings. He brings belief.”

It was Tuchel’s first night game under the Stamford Bridge lights in front of fans and the first time the stadium had been packed for a European fixture since February of last year. So much has happened since then, not least Chelsea exploding from nowhere to claim their second Champions League, and it feels crazy to reflect that the home supporters were unable to play any role in it – apart from when 2,000 were allowed in for the final group match against Krasnodar last December.

The pre-match presentations were a nod towards the history the team had made. There were Uefa awards for Tuchel, Jorginho, Edouard Mendy and N’Golo Kanté but this was all about putting a sure foot forward, ensuring that the defending Russian Premier League champions could not spring an upset.

Chelsea know what they are: the champions of Europe. And their support repeatedly let everyone know. It feels sweet and it helps to create an aura. Zenit, though, were not intimidated. They were compact throughout in their 5-4-1 system and they flickered on the counter, even if it is one thing to enjoy encouragement against this Chelsea defence, which marries pace, positioning and physicality, and quite another to create anything clear-cut.

Chelsea had to be patient and Tuchel battled to suppress the frustration he felt in the first half when his team could not connect in the final third. Jorginho tried to play through the lines, to make something happen, while Mateo Kovacic was busy but Chelsea did not work the Zenit goalkeeper, Stanislav Kritsyuk. Mason Mount missed a few passes and it did not happen for Hakim Ziyech.

Chelsea needed a spark and Antonio Rüdiger, a massive presence throughout, provided it on 50 minutes with a storming run from inside his own half that finished with a snatched shot wide.

Chelsea’s intensity was greater in the second half, as was their intent, and they had a few half-chances although Zenit felt they retained a puncher’s chance on the break. There was a sharp intake of breath inside the stadium when Claudinho sent Azmoun away before Rüdiger got back to tackle. And it was not for the first time.

Enter Lukaku. The centre-forward would almost add a late second, only to be snuffed out by Douglas Santos. One was enough.