Branislav Ivanovic endured a personal nightmare as Swansea completed a 2-0 away win over Chelsea in their League Cup semi-final first leg at Stamford Bridge.
Michael Laudrup's side were handed a couple of belated Christmas presents by Rafael Benitez's team as Ivanovic was left badly at fault when Michu (39) and substitute Danny Graham (90+1) netted the goals that handed Swansea their first win at Chelsea in 87 years.
It leaves the Welsh side on the cusp of an unexpected final spot at Wembley after the opening leg of the club's first major semi-final in 49 years. Chelsea dominated possession, but could not find a way through a visiting defence that was expertly organised by captain Ashley Williams.
The return leg will be staged at Swansea's Liberty Stadium on January 23. The winners will face Bradford City or Aston Villa in the final. League Two Bradford lead 3-1 after their first leg at Valley Parade 24 hours earlier.
Swansea moved ahead with their only meaningful attack of the first period when Ivanovic needlessly shed possession for Jonathan de Guzman to play in Michu.
The Spanish forward had plenty of work to do, but was the calmest man inside the London ground as he planted the shot beyond home goalkeeper Ross Turnbull from just inside the Chelsea area for his 16th goal of the season.
Ivanovic was then guilty of another wretched moment when he was short with a back-pass as Graham skipped clear of Turnbull in stoppage time before tucking the ball into the vacant net as the home fans looked on in a state of disbelief.
Chelsea had appealed for a penalty seconds earlier when Demba Ba - who had replaced the disappointing Fernando Torres - appeared to collide with Swansea goalkeeper Gerhard Tremmel.
Referee Anthony Taylor decided otherwise before booking Ba for diving, which appeared a harsh call in the circumstances.
Ba was out of luck again after Swansea had snared their second goal when he managed to thump a shot into the net from a Frank Lampard flick on only to see the flag of an assistant lineman render his effort in vain.
The last team to overturn a two-goal deficit from the first leg of a League Cup semi-final was Aston Villa in 1994. Villa lost 3-1 to Tranmere, but they needed penalties to progress after a 3-1 win in the second leg of that tie.
Chelsea will look to their efforts in the first half here to at least know they can trouble Swansea.
Ramires saw Tremmel hold his prodded shot on 11 minutes before the goalkeeper was called upon to halt a Juan Mata effort after some neat play by Eden Hazard.
Oscar then sent Mata scampering into the visiting box, but he could only swipe a shot wide of the home goal. The half ended with Tremmel beating out an Ivanovic hit from distance as Chelsea somehow headed for the dressing room trailing to Michu's goal.
Tremmel made competent saves from David Luiz and Lampard in the second half while he was alert to hold a Ba header, but Swansea cut an assured lot as the game wore on with Chelsea appearing to run out of ideas.
Chelsea have failed to score in four of Rafa Benitez's six home matches as manager, but scored 14 goals in the other two which hints at a lack of consistency.
MAN OF THE MATCH
Ashley Williams (Swansea): Another outstanding performance by the Welsh international at the heart of a well-organised Swansea defence.
Chelsea: Turnbull 6, Azpilicueta 6, Ivanovic 4, Cahill 5, Cole 5, David Luiz 6, Ramires 6, Mata 7, Oscar 6, Hazard 6, Torres 5. Subs: Lampard 6, Marin 6, Demba Ba 6
Swansea: Tremmel 7, Rangel 7, Chico 7, Williams 9, Davies 7, Routledge 6, Britton 6, de Guzman 7, Ki 7, Hernandez 6, Michu 8. Subs: Tiendalli 7, Graham 7