London – Told all week that the game against Tottenham Hotspur would define the title race, the moment when the balance of power would swing in Spur’s way, a less durable team than Chelsea might have responded to Dele Alli’s splendid FA Cup semi-final equalizer shortly after half-time by crumbling. On another day, it would have been in the script. Tottenham, brilliant and refreshing, were on top. They were surely going to reach the FA Cup final. Then they were going to win the league.

But even when Chelsea were on the ropes, to doubt them in the moments before Eden Hazard’s decisive cameo was to fail to understand them. There is something about this club. For all the nouveau riche jibes and accusations of artificially bought success that have flown their way ever since Roman Abramovich spotted Stamford Bridge from his helicopter 14 years ago, and for all the managerial changes and bouts of dressing-room insurrection that have often made them look like a textbook case of everything that is wrong with modern football, there is also an inner toughness running through Chelsea that allows them to rebel, stand firm, trust in themselves and emerge a little battered, a little bruised, but ultimately unscathed.

It dates back to the first José Mourinho era, when the Portuguese arrived at Stamford Bridge and infused an underachieving squad with an unforgiving winning mentality. Neutrals had admired the Chelsea of Gianfranco Zola, an entertaining, infuriating team that saved its best for the cups. Mourinho replaced Claudio Ranieri, one of football’s nice guys and a loveable loser until his Leicester City adventure. The perception of Chelsea changed. You could respect their pragmatic football without particularly liking it.

But it brought them enviable success, even after Mourinho’s first acrimonious departure. A macho core of Petr Cech, John Terry, Frank Lampard and Didier Drogba kept Chelsea’s fighting spirit intact, with their refusal to lie down best summed up by that scrapping Champions League triumph in 2012.

The Chelsea of 2017 do have a different, softer feel in places. They are not quite as physically imposing. Give or take a Diego Costa or a David Luiz, their players look less likely to instigate a 22-man brawl. Time has taken its toll on the old guard. Branislav Ivanovic and Mikel John Obi said goodbye in January, while Terry will leave at the end of the season.

The club captain’s departure will mark the end of a wildly successful era for Chelsea. Yet even now, in a period of change, they are the toughest of nuts to crack. They might not be the most thrilling side in England but they are by far the most efficient and controlled.

It helps that they hired Antonio Conte last summer, a manically driven winner who is yet to meet a technical area that can hold him. Chelsea were supposedly at a low ebb before kick-off against Tottenham in the FA Cup semi-final on April 22. Well beaten by Manchester United in their previous match, they were without the unwell Gary Cahill, while Conte decided to rest Costa and Hazard before the visit of Southampton later that week. Four points clear of Tottenham with six league matches left, it was a big gamble from the Italian. There were rare starts for Nathan Aké and Michy Batshuayi. And there were two goals in the first half for Willian, Hazard’s replacement. Antonio knows.

“If you ask me last season, after our 10th place, this season you stay on top of the table and you reach the FA Cup final, but you didn’t change a lot, I think it’s great,” Conte said. “But we must continue to work and improve. If you compare our team with Tottenham’s, Pochettino has worked for three years.”

There is, of course, something important to point out here. As Chelsea close in on their first Double since Carlo Ancelotti’s 2010 vintage, it does feel faintly ridiculous to portray them as underdogs punching above their weight. Sure, they finished 10th last season. But that was almost as much of a freak occurrence as Leicester winning the league and Chelsea reacted in the summer by hiring a former Juventus and Italy manager and spending £120m on four players. Mauricio Pochettino sighed when he spoke about Chelsea bringing on Hazard, Costa and Cesc Fàbregas when the score was 2-2. From Fàbregas’s late corner, Hazard scored to give Chelsea the lead before going on to create Nemanja Matic’s stunning clincher.

All the same, it is easy to understand why Conte prefers to talk of Chelsea as unlikely champions, arguing that his job has been much tougher than Ancelotti’s. That siege mentality came in handy after Christian Eriksen had created two fine equalizers for Alli and Harry Kane.

“I think that now at Chelsea we are in a period of transition,” Conte said. “In this season, we have lost Ivanovic, Mikel and next season we lose John Terry. We are talking about players who wrote the history of this club. They won a lot. Now we have to find the right substitutes for these players and then work to put these players at the same level.

“I think that in the period with Carlo – and Carlo is the best Italian coach, I have great respect for him – he arrived at Chelsea when they had a really strong squad. Now we are building. We are building something important. We need time.”

Patience is a luxury rarely afforded to Chelsea’s managers. All that chopping and changing has made it harder to orchestrate a transition. Yet as a new generation emerges, the moment when Hazard’s drive beat Hugo Lloris was a reminder of the killer instinct of old.

The Guardian Sport