Chelsea missed a host of chances as Championship side Middlesbrough earned a slender advantage in the first leg of their Carabao Cup semi-final at a boisterous Riverside Stadium.
Hayden Hackney scored the only goal, opening up his body to slide home an Isaiah Jones cross in the first half.
Cole Palmer missed three presentable opportunities as Chelsea, who had 18 shots, but only five on target, failed to score for the sixth time this season.
"It is difficult to talk about our performance," Chelsea boss Mauricio Pochettino told BBC Radio 5 Live.
"We created some chances, but we were not clinical enough. This first half of the season, we have created some chances, but we have not won because we were not clinical enough - we must keep pushing."
Chelsea, who sit 10th, have missed 40 big chances - as defined by statistical analysts Opta - in the Premier League so far this season, the second-highest total in the top flight, behind Liverpool (41).
They are looking to bring in a forward in the next two transfer windows but it is unlikely they will be spending the huge money needed to land Brentford's Ivan Toney or Victor Osimhen from Napoli.
Boro, who have lost six games at home in the Championship this season, can now dream of the reaching the final, 20 years on from winning the tournament.
They came into the came with 12 first-team players unavailable and lost striker Emmanuel Latte Lath and left wing-back Alex Bangura to injuries in the opening 20 minutes.
"The way they stuck together was very special and makes me proud to see that," Middlesbrough manager Michael Carrick told Sky Sports.
"We had to show a bit of everything and I'm buzzing. I know what it means to everyone, it was fantastic for me to see so many people so happy and it's pretty special.
"We will be ready for the second leg and attack the game and see what happens. Football is about dreaming and things happen like tonight which probably shouldn't happen. We go there with our eyes wide open and know what to expect, but it won't stop us trying."
The second leg takes place on Tuesday, 23 January at 19:45 GMT.
Liverpool face Fulham in the other semi-final, with the final taking place at Wembley on Sunday, 25 February.
Palmer struggles as injury-ravaged Boro battle
Boro started well and Latte Lath had capitalised on a poor Levi Colwill header to get in behind the Chelsea defence and was caught on the ankle by Axel Disasi, but the referee waved away penalty appeals and there is no video assistant referee system in operation at this stage of the competition.
Palmer's first big miss came when experienced captain Jonny Howson gifted him the ball, 30 yards from the Boro goal. He drifted towards goals and looked to place in the corner but dragged wide.
Minutes later, Boro took the lead when Jones got in behind the Chelsea defence and twisted and turned Colwill, who struggled throughout, before his cross was turned in by Hackney, who had drifted off Moises Caicedo.
The goal was met with rapturous noise inside the Riverside, but the game soon returned to the familiar pattern of Chelsea attacks with Palmer turning over the bar after Enzo Fernandez's effort was spilled by Tom Glover.
Former Manchester City forward Palmer then saw another effort saved as he chopped in from the right, while Caicedo had a long-range effort fizz wide.
Chelsea dominated large periods of the second half too but failed to create any clear-cut opportunities, with a Noni Madueke header, which was comfortably saved, the closest they came.
Their recent investment can be highlighted by the fact that only two of the squad who played in an FA Cup tie against Boro in January 2022 are at the club this season, and while summer signing Palmer has been a shining light so far, this was a difficult night for the 21-year-old.
Pochettino's side had scored in their past six games but must improve in the second leg if the Argentine boss is going to retain a chance of winning his first trophy in England at the end of February.
Among Premier League sides, only Bournemouth and Nottingham Forest (32 each) have lost more games than Chelsea (31) in all competitions since the start of last season, while no team have lost more away from home than the Blues (21) during that period.
Thiago Silva and a handful of other players approached some angry Chelsea fans at full-time, but Pochettino insisted "they were fantastic with us" and that his fans were "reacting to Middlesbrough fans", with his players "trying to calm our fans".
Player of the match
HackneyHayden Hackney
with an average of 8.39Middlesbrough
Squad number7Player nameHackney
Average rating8.39
Squad number11Player nameJones
Average rating8.28
Squad number16Player nameHowson
Average rating8.06
Squad number25Player nameCrooks
Average rating7.87
Squad number27Player nameEngel
Average rating7.85
Squad number5Player nameClarke
Average rating7.82
Squad number3Player namevan den Berg
Average rating7.81
Squad number4Player nameBarlaser
Average rating7.78
Squad number6Player nameFry
Average rating7.75
Squad number19Player nameCoburn
Average rating7.66
Squad number23Player nameGlover
Average rating7.50
Squad number24Player nameBangura
Average rating7.41
Squad number9Player nameLatte Lath
Average rating7.34
Chelsea
Squad number28Player namePetrovic
Average rating4.49
Squad number6Player nameThiago Silva
Average rating4.43
Squad number23Player nameGallagher
Average rating4.18
Squad number42Player nameGilchrist
Average rating4.03
Squad number27Player nameGusto
Average rating3.87
Squad number2Player nameDisasi
Average rating3.65
Squad number11Player nameMadueke
Average rating3.65
Squad number8Player nameFernández
Average rating3.49
Squad number26Player nameColwill
Average rating3.38
Squad number25Player nameCaicedo
Average rating3.28
Squad number7Player nameSterling
Average rating3.18
Squad number19Player nameBroja
Average rating2.99
Squad number10Player nameMudryk
Average rating2.96
Squad number20Player namePalmer
Average rating2.12
Line-ups
Middlesbrough
Formation 3-4-2-1
23Glover
3van den Berg6Fry27Engel
11Jones4Barlaser16Howson24Bangura
25Crooks7Hackney
9Latte Lath
- 23Glover
- 3van den Berg
- 6Fry
- 27Engel
- 11JonesBooked at 90mins
- 4Barlaser
- 16Howson
- 24BanguraSubstituted forClarkeat 20'minutes
- 25Crooks
- 7Hackney
- 9Latte LathSubstituted forCoburnat 5'minutes
Substitutes
- 5Clarke
- 14Gilbert
- 19Coburn
- 28O'Brien
- 32Jones
- 48Kavanagh
- 49McCabe
- 52Bridge
- 57Bilongo
Chelsea
Formation 4-2-3-1
28Petrovic
27Gusto2Disasi6Thiago Silva26Colwill
25Caicedo8Fernández
11Madueke23Gallagher7Sterling
20Palmer
- 28Petrovic
- 27GustoSubstituted forGilchristat 90'minutes
- 2Disasi
- 6Thiago Silva
- 26ColwillBooked at 30mins
- 25Caicedo
- 8FernándezSubstituted forBrojaat 63'minutes
- 11MaduekeSubstituted forMudrykat 63'minutes
- 23Gallagher
- 7Sterling
- 20Palmer
Substitutes
- 10Mudryk
- 13Bettinelli
- 19Broja
- 36de Souza Eugênio
- 42Gilchrist
- 47Bergström
- 51Williams
- 68Golding
Referee:Samuel Barrott
Attendance:32,097
Match Stats
Live Text
Match ends, Middlesbrough 1, Chelsea 0.
Second Half ends, Middlesbrough 1, Chelsea 0.
Foul by Armando Broja (Chelsea).
Jonny Howson (Middlesbrough) wins a free kick in the defensive half.
Corner, Chelsea. Conceded by Hayden Hackney.
Isaiah Jones (Middlesbrough) is shown the yellow card.
Foul by Moisés Caicedo (Chelsea).
Josh Coburn (Middlesbrough) wins a free kick in the defensive half.
Delay over. They are ready to continue.
Substitution, Chelsea. Alfie Gilchrist replaces Malo Gusto.
Delay in match because of an injury Matt Crooks (Middlesbrough).
Attempt blocked. Conor Gallagher (Chelsea) right footed shot from outside the box is blocked.
Offside, Middlesbrough. Josh Coburn is caught offside.
Attempt missed. Axel Disasi (Chelsea) left footed shot from the centre of the box is close, but misses to the left following a corner.
Corner, Chelsea. Conceded by Jonny Howson.
Attempt missed. Raheem Sterling (Chelsea) right footed shot from the centre of the box is close, but misses the top right corner. Assisted by Mykhailo Mudryk.
Attempt saved. Mykhailo Mudryk (Chelsea) right footed shot from outside the box is saved in the centre of the goal. Assisted by Raheem Sterling.
Delay over. They are ready to continue.
Delay in match because of an injury Malo Gusto (Chelsea).
Attempt missed. Daniel Barlaser (Middlesbrough) right footed shot from outside the box is too high. Assisted by Hayden Hackney following a fast break.
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