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.
