Cesc Fabregas scored twice in extra time as Chelsea came from two goals down to beat 10-man Leicester and reach the EFL Cup fourth round.
Shinji Okazaki’s header and instinctive strike put Leicester in control, but Gary Cahill nodded in before the break.
Cesar Azpilicueta equalised with a long-range strike before Foxes defender Marcin Wasilewski was sent off.
Fabregas’ clipped finish put Chelsea ahead early in the first extra period and his powerful strike sealed the win.
It had been a pulsating cup tie – particularly in the second half – with chances at both ends but Wasilewski’s dismissal for elbowing Diego Costa ultimately proved costly.
Chelsea deserved the win in the end but Claudio Ranieri will perhaps be buoyed by his side’s performance for much of the game, with glimpses of their incisive attacking and compact defending that helped them win the title last season on show.
Slick in attack, sloppy in defence
Chelsea have scored in every game this season and their attacking strength came to the fore towards the end of this game.
However, their defensive frailties should continue to be a cause for concern for boss Antonio Conte.
Only once in their previous 14 games in all competitions have the Blues managed to keep a clean sheet – and they were needlessly exposed in the first half of this game.
Azpilicueta’s awkward touch when trying to deal with Ahmed Musa’s cross allowed Ozaki to nip in and flick a header over Asmir Begovic, and the Foxes forward then profited from a static backline to score a second.
“Chelsea’s decision-making when under pressure was poor,” BBC Radio 5 live summariser and former Leicester defender Gerry Taggart said.
“We are talking about basics. Clear the ball first, then ask questions. Some of Chelsea’s decision making was abysmal.”
Fabregas has been unable to establish himself under Conte, with the Spanish midfielder restricted to appearances from the bench in the Premier League so far this season.
His only two starts for the Italian have come in the EFL Cup – but he was arguably the stand out player for the Blues in this game.
The 29-year-old has been criticised for a lack of goals in the past – he scored six last season and five the campaign before – but took both his finishes well against the Foxes and also provided the assist for Cahill’s goal.
His was a performance to give Conte food for thought.
Man of the match – Cesc Fabregas
What they said
Leicester boss Claudio Ranieri: “I am happy with our performance until 2-0. We started very well and we were in control of the match. Chelsea had more possession but we were very smart and clever to score twice.
“The key was when we conceded a goal on the last corner of the half, we lost two dangerous men, Luiz and Cahill.
“In extra time I said stay together and close the space, but Fabregas scored and it was very difficult for us.”
Chelsea boss Antonio Conte: “I want to win and my choice is done to win. I don’t care what their name is. For me it’s important when I call someone to play and they show me my choice is not wrong.
“I am pleased for Cesc because he played a good game, he showed me great commitment in the training sessions. I am satisfied when I see this behaviour. This is the right way and I want my players to create good competition and, game by game, give me the opportunity to choose the best line-up.”
The stats you need to know
Chelsea have come back to win from two goals down for the first time in a game since August 17 2002 v Charlton (3-2).
Gary Cahill has scored five goals in all competitions since August 2015; three more than any other Chelsea defender.
Shinji Okazaki netted his first ever brace for Leicester City after failing to score in his previous 13 games for the Foxes.
Leicester return to Premier League action at the weekend with a tricky away fixture at Manchester United on Saturday. Chelsea, meanwhile, will look to bounce back from a 2-1 defeat by Liverpool last time out in the league when they travel to Arsenal, also on Saturday.
Match ends, Leicester City 2, Chelsea 4.
Second Half Extra Time ends, Leicester City 2, Chelsea 4.
Diego Costa (Chelsea) wins a free kick on the left wing.
Foul by Danny Simpson (Leicester City).
Offside, Chelsea. Nathaniel Chalobah tries a through ball, but Cesc Fàbregas is caught offside.
Attempt missed. Cesc Fàbregas (Chelsea) right footed shot from the centre of the box is too high. Assisted by Marcos Alonso with a cross.
David Luiz (Chelsea) wins a free kick in the defensive half.
Foul by Jamie Vardy (Leicester City).
David Luiz (Chelsea) is shown the yellow card for a bad foul.
Foul by David Luiz (Chelsea).
Jamie Vardy (Leicester City) wins a free kick on the right wing.
Second Half Extra Time begins Leicester City 2, Chelsea 4.
First Half Extra Time ends, Leicester City 2, Chelsea 4.
Foul by Eden Hazard (Chelsea).
Daniel Drinkwater (Leicester City) wins a free kick in the defensive half.
Ben Chilwell (Leicester City) is shown the yellow card for a bad foul.
César Azpilicueta (Chelsea) wins a free kick in the defensive half.
Foul by Ben Chilwell (Leicester City).
Attempt blocked. Victor Moses (Chelsea) right footed shot from outside the box is blocked. Assisted by Eden Hazard.
Corner, Chelsea. Conceded by Ron-Robert Zieler.
Attempt saved. Nemanja Matic (Chelsea) left footed shot from outside the box is saved in the bottom left corner.
Goal! Leicester City 2, Chelsea 4. Cesc Fàbregas (Chelsea) left footed shot from the centre of the box to the high centre of the goal following a corner.
Corner, Chelsea. Conceded by Ben Chilwell.
Goal! Leicester City 2, Chelsea 3. Cesc Fàbregas (Chelsea) right footed shot from the centre of the box to the top left corner. Assisted by Eden Hazard.
First Half Extra Time begins Leicester City 2, Chelsea 2.
Second Half ends, Leicester City 2, Chelsea 2.
Attempt missed. Diego Costa (Chelsea) right footed shot from a difficult angle on the right is close, but misses to the right. Assisted by César Azpilicueta.
Attempt missed. Victor Moses (Chelsea) right footed shot from outside the box is close, but misses to the left following a corner.
Substitution, Leicester City. Daniel Amartey replaces Demarai Gray.
Corner, Chelsea. Conceded by Danny Simpson.
Second yellow card to Marcin Wasilewski (Leicester City) for a bad foul.
Diego Costa (Chelsea) wins a free kick on the left wing.
Foul by Marcin Wasilewski (Leicester City).
Substitution, Chelsea. Eden Hazard replaces Pedro.
Attempt missed. David Luiz (Chelsea) right footed shot from outside the box is just a bit too high from a direct free kick.
Daniel Drinkwater (Leicester City) is shown the yellow card for a bad foul.
Diego Costa (Chelsea) wins a free kick in the attacking half.