Match ends, Sunderland 2, Manchester United 1.
Manchester United lost a third game in a row for the first time since 2001 as Sunderland took a one-goal lead into the Capital One Cup semi-final second leg.
Fabio Borini's second-half penalty, after Tom Cleverley was controversially penalised for a challenge on Adam Johnson, was the decisive goal.
Ryan Giggs' own goal in first-half injury time had put Sunderland ahead.
Nemanja Vidic levelled with a towering header soon after the restart.
But Sunderland earned their first win over United since 2000, a run of 21 matches, when Liverpool loanee Borini kept his calm from the penalty spot after substitute Johnson went down under Cleverley's challenge.
United were furious with the decision but Cleverley's attempt to win the ball looked rash as Johnson cut across him, leaving United chasing the game ahead of the second leg at Old Trafford on 22 January.
The defeat deepens the woes of manager David Moyes after United's FA Cup hopes were ended by Swansea on Sunday, while their hopes of retaining the Premier League title appear to be disappearing, with leaders Arsenal 11 points clear.
United have won the League Cup three times in the past decade, in 2006, 2009 and 2010, but a premature exit this time around would leave them relying almost solely on the Champions League to rescue their campaign.
Sunderland's own cup form provides a welcome distraction to their perilous Premier League situation. Gus Poyet's men are bottom of the table but went into the tie looking for a sixth straight home win in knockout competitions this season.
The hosts' backline had a few familiar faces for United fans with Wes Brown and John O'Shea, who both made more than 300 appearances for the visitors, in the centre of defence and another former Red, Phil Bardsley, at right-back.
And, despite almost 18,000 empty seats at the Stadium of Light, they looked relatively untroubled until, out of the blue, Giggs saw a 20-yard shot cannon of the crossbar via a slight deflection after a driving run from the 40-year-old through midfield.
Adnan Januzaj, at 18 less than half the age of Giggs, scored both goals on his full debut in a 2-1 win at Sunderland in October.
He thought he had broken the deadlock for United when he calmly slotted home after his initial shot had been blocked by Giggs, only for the Welshman to be correctly flagged offside.
Giggs' evening got even worse in first-half injury time when he diverted the ball into his own net as he tried to prevent Bardsley from tapping home, after Brown had volleyed Sebastian Larsson's deep free-kick back across goal.
Sunderland, losing League Cup finalists in 1985, were not ahead for long as Vidic towered above Brown and O'Shea to head Cleverley's corner into the net soon after the restart.
With the game swinging from end to end, Larsson's volley was parried away by United goalkeeper David De Gea before the hosts again took the initiative in controversial circumstances.
Referee Andre Marriner looked to have waved play on when Johnson tumbled under Cleverley's challenge in the penalty area but, after being advised by his assistant that contact had been made, he pointed to the spot. Borini made no mistake from 12 yards.
United poured forward unsuccessfully in a search of a leveller but, with away goals counting double after extra-time in the second leg, Sunderland will know a lot of hard work is ahead before they book their place at Wembley - despite ending United's seven-match unbeaten run away from home.
Sunderland manager Gus Poyet:
"We know how difficult it will be at Old Trafford. We know how good they are, but to have a chance you need to take advantage of the home leg. We scored the two goals to give us that little advantage.
"We are going to try and play our game at Old Trafford. It is not a bad result but of course it is going to be very difficult.
"The most important thing of the day is to have Adam Johnson back to his best, he will be crucial for us."
- 25MannoneBooked at 90mins
- 2BardsleyBooked at 60mins
- 4Ki Sung-yueng
- 9FletcherSubstituted forAltidoreat 72'minutesBooked at 77mins
- 23GiaccheriniBooked at 51minsSubstituted forJohnsonat 56'minutes
- 1de Gea
- 2da SilvaBooked at 63mins
- 3EvraBooked at 24mins
- 23CleverleySubstituted forFletcherat 74'minutes
- 6EvansSubstituted forSmallingat 61'minutesBooked at 83mins
- 25A ValenciaSubstituted forHernándezat 87'minutes
- Andre Marriner
- Shots on Target
Second Half ends, Sunderland 2, Manchester United 1.
Vito Mannone (Sunderland) is shown the yellow card.
Attempt missed. Adam Johnson (Sunderland) right footed shot from outside the box misses to the right.
Foul by Michael Carrick (Manchester United).
Fabio Borini (Sunderland) wins a free kick on the left wing.
Attempt missed. Michael Carrick (Manchester United) right footed shot from outside the box misses to the right. Assisted by Darren Fletcher following a corner.
Corner, Manchester United. Conceded by Fabio Borini.
Attempt blocked. Adnan Januzaj (Manchester United) left footed shot from the right side of the box is blocked. Assisted by Ryan Giggs.
Corner, Manchester United. Conceded by Wes Brown.
Substitution, Manchester United. Javier Hernández replaces Luis Antonio Valencia.
Attempt saved. Danny Welbeck (Manchester United) header from the centre of the box is saved in the centre of the goal.
Rafael (Manchester United) wins a free kick in the defensive half.
Foul by Marcos Alonso (Sunderland).
Chris Smalling (Manchester United) is shown the yellow card for hand ball.
Hand ball by Chris Smalling (Manchester United).
Danny Welbeck (Manchester United) wins a free kick in the defensive half.
Foul by Wes Brown (Sunderland).
Attempt missed. Patrice Evra (Manchester United) left footed shot from outside the box misses to the right. Assisted by Darren Fletcher.
Attempt missed. Adnan Januzaj (Manchester United) right footed shot from the centre of the box is just a bit too high. Assisted by Luis Antonio Valencia with a cross.