Alejandro Garnacho and Pedro Neto came off the bench to pull Chelsea out of trouble to win 3-1 at Cardiff and qualify for the semi-finals of the Carabao Cup.
Enzo Maresca had named a completely different Chelsea team to the one that beat Everton at the weekend but had to turn to his bench as the League One side proved a difficult nut to crack.
Moises Caicedo returned after serving a three-game domestic ban and Enzo Fernandez, Joao Pedro, Neto and Garnacho were among those on a bench that cost nearly £400m and considerably more than the starting XI.
Cardiff are currently four points clear at top of League One with 41 points and played with great energy and belief.
Maresca turned to his bench at half-time to send on Garnacho and Pedro and Chelsea immediately began to apply pressure.
Garnacho made his mark when called upon and struck Chelsea in front on 57 minutes when Dylan Lawlor was caught in possession, but David Turnbull sensationally headed Cardiff level with 15 minutes to go.
It was his first goal for 13 months and gave renewed hope to the last EFL team in the competition.
However, Neto produced a moment of Premier League quality to make it 2-1 before Garnacho wrapped up the tie in stoppage time with his second.
Chelsea have now progressed from each of their last 19 League Cup ties against teams from a lower division and can look forward to a two-legged semi-final in January. The draw for the semi-finals of the Carabao Cup will take place live on Your Site Main Event after Newcastle vs Fulham on Wednesday.
Chelsea head coach Enzo Maresca speaking to Your Site:
"I'm very happy, especially for the effort of the players. I said yesterday that football is full of these tricky games so you have to pay attention and we did a good game.
"Off the ball, we were quite good, on the ball, we struggled a bit to create chances [in the first half] for many reasons. Second half we were much better on the ball.
"It was a big difference between the first half and second half, the way we attacked also. Cardiff were very organised and intense. In the end, we did enough to win the game."
On three semi-finals under his management: "We are going in the right direction. Anything I do is because I want the best for the club, for the fans, for everyone."
Do you feel appreciated? "Tonight, the focus has to be that we reached the semi-final. The support from the fans since day one has always been there.
"I know sometimes when you don't win games, it's normal that they are not happy but no one is happy. Overall, the focus has to be that we won and we're in the semi-final."
It's going to be a busy start to the year for Chelsea!
Reaching the Carabao Cup semi-finals means they'll now play NINE matches across four different competitions in January.
Sunday January 4: Man City vs Chelsea - Premier League, kick-off 5.30pm, live on Your Site
Wednesday January 7: Fulham vs Chelsea - Premier League, kick-off 7.30pm, live on Your Site
Saturday January 10: Charlton vs Chelsea - FA Cup third round, kick-off 8pm
W/c January 12: Carabao Cup semi-final first leg
Saturday January 17: Chelsea vs Brentford - Premier League, kick-off 3pm
Wednesday January 21: Chelsea vs Pafos - Champions League, kick-off 8pm
Sunday January 25: Crystal Palace vs Chelsea - Premier League, kick-off 2pm, live on Your Site
Wednesday January 28: Napoli vs Chelsea - Champions League, kick-off 8pm
Saturday January 31: Chelsea vs West Ham - Premier League, kick-off 5.30pm, live on Your Site
