The visitors deservedly took the lead only a minute after the interval when Frank Lampard scrambled his effort over the line after a great cross from Cesar Azpilicueta.
However, the West London club were wasteful in front of goal, with Kevin De Bruyne, Frank Lampard and Demba Ba all missing glorious chances to put the game beyond doubt.
And Jose Mourinho's men were made to pay when substitute Fabio Borini, who played for Chelsea as a teenager, slid home from a tight angle in the closing moments to send the game to extra time.
The fairy tale continued for the Mackems when another substitute, Ki-Sung Yueng, placed his effort past Mark Schwarzer in the 118th minute to hand Gus Poyet a shock victory.
After a tense encounter, here are six things we can take from the game after Sunderland booked their place in the semifinal.
A win? Gus Poyet would take that at the moment.
A win against his old club to take them through to the League Cup semifinal? Priceless.
The Uruguayan coach may see his side struggling in the Premier League, with the Black Cats currently sitting at rock bottom. But there's a slight reprieve for Poyet in the cup now and the chance to win a trophy.
Even when his side went 1-0 down to Chelsea, they refused to give up. Poyet never looked like he was prepared to take defeat and responded with two substitutions in Fabio Borini and Ki-Sung Yueng, and he must take the credit, with both scoring to complete an amazing turnaround.
Poyet will now be hoping they can transfer their cup form over to the league, where they face an uphill struggle to stay in the Premier League, but the signs are encouraging for sure.
For all of his trophies and the reputation as one of the most gifted tacticians in world football, Jose Mourinho must be struggling to process what is happening at Stamford Bridge.
Their sixth defeat in all competitions is something unusual for Mourinho, for he is simply not used to the idea of losing.
But whereas the players are, and should always, take responsibility for their latest shock defeat, Mourinho should too.
He was tactically outwitted by Gus Poyet, who saw two of his three substitutes score in the latter stages of the game, while Mourinho's tactical gambles such as Michael Essien and Demba Ba looked way off the pace.
Another away game, another frustrating away defeat for Chelsea. Where next?
Many will not be surprised about this, and in fact, some might be more surprised that Willian hasn't featured as regularly as he might have done, considering he cost a modest £30 million.
But the Brazilian has taken his chance in the first-team brilliantly and has firmly established himself as one of Chelsea's top performers in recent weeks.
The 25-year-old can play anywhere in the attacking midfield, and against Sunderland, he was given the opportunity to explore the freedom that the No. 10 role brings, as opposed to playing on either the left or right.
Of course, he will come up against tougher opposition, but the former Shakhtar playmaker showed some neat touches and generally looked lively in the final third. Not only that, but he worked back to help out the defence.
He couldn't quite work an opening for his team-mates, but he deserves some recognition for another dynamic display.
Kevin De Bruyne's future at Chelsea is in doubt after Bayer Leverkusen repeated their desire to sign the Belgian youngster, per Sky Sports' Kristof Terreur, but after his impressive display against Sunderland, will he be leaving after all?
The 22-year-old put in his best performance in a Chelsea shirt, with the versatile midfielder always looking to get the ball forward into dangerous areas.
He played slightly more advanced, although the former Genk winger was forced to track back and defend in more desperate times.
But unlike what we usually see from a player who wants to leave their respective club, De Bruyne showed no sign of sulking or drop in intensity and played the full 90 minutes.
Jose Mourinho still might decide to let him go after continuous speculation, but it seems the Blues have a promising player among their ranks if he decides to stay.
Fabio Borini owes a small bit of his career to Chelsea, who signed him from Bologna at the age of 16. But what has happened since then perfectly captures how inconsistent the Italian can be.
A loan spell at Swansea propelled him into the limelight before joining Roma on a free transfer in 2011 before joining Liverpool in a £12 million deal.
Just one Premier League goal later and he was shipped off on loan to Sunderland, where again he has failed to live up to the high standards expected of him.
But the 22-year-old certainly felt he had the chance to prove that Chelsea were wrong not to give him a chance as a youngster, and his strike from a tight angle helped Sunderland on their way to a vital win.
Borini's celebration was emphatic, there was no affiliation in his heart to the Blues, evidently. And why should he? He is a footballer simply doing his job, and after being sent on as a late substitute, he fulfilled his role as "super sub."
Had Chelsea got their man in the summer, it's likely that Jose Mourinho would be cruising towards his third Premier League title.
Rooney, on the other hand, has scored eight on his own with six assists to go with that for the Red Devils. Mourinho shudders at the thought of what they could have achieved together.
Samuel Eto'o was the choice for the lone forward role but was largely isolated throughout. And when the Cameroon veteran did get the ball in a good position, he wasted the chance.
Demba Ba appeared to be out of sorts when he replaced Eto'o, and the indication suggests he could well leave if a new striker is signed in January, with the Senegal striker kept quiet by Wes Brown and John O'Shea.
Chelsea are okay in the league but they cannot afford any more slip-ups like this in the cup competitions.