At a rowdy Riverside Stadium, Chelsea squandered a tonne of opportunities as Championship team Middlesbrough gained a narrow advantage in the first leg of their Carabao Cup semi-final.
In the first half, Hayden Hackney scored the game's lone goal by extending his body and sliding in an Isaiah Jones cross.
Chelsea failed to score for the sixth time this season as Cole Palmer squandered three good chances.
The second leg starts at 19:45 GMT on Tuesday, January 23.
The other semi-final pits Liverpool against Fulham, while the final is scheduled for Sunday, February 25, at Wembley.
Palmer battles through an injury-ravaged Boro
Twelve first-team players were out for both teams going into the match, and Boro manager Michael Carrick's predicament got worse when left wing-back Alex Bangura and striker Emmanuel Latte Lath were substituted off in the first 20 minutes.
Axel Disasi caught Latte Lath on the ankle as he escaped the Chelsea defense thanks to a poorly executed header by Levi Colwill. However, the referee rejected penalty requests, and at this point in the competition, there is no video assistant referee system in place.
Palmer's first major error occurred thirty yards from the Boro goal when seasoned captain Jonny Howson gave him the ball. He slid in the direction of the goals, trying to situate himself in the corner but missing.
A few minutes later, Hackney, who had slipped off Moises Caicedo, turned in Jones' cross, giving Boro the lead. Jones had gotten in behind the Chelsea defense and twisted and turned Colwill, who had struggled throughout.
The crowd at the Riverside was ecstatic when the goal was scored, but Chelsea's trademark attacking style was quickly resumed when Palmer saved a penalty kick when Tom Glover mishandled Enzo Fernandez's attempt.
After that, Caicedo's long-range attempt went wide, and former Manchester City forward Palmer's attempt was saved again as he cut in from the right.
Even though Chelsea had the better of the second half for long periods, they were unable to convert any clear-cut chances; the closest they got was a header from Noni Madueke that was easily saved.
Only two of the players who participated in an FA Cup match against Boro in January 2022 are still with the team this season, highlighting their recent investment. Although summer addition Palmer has performed admirably thus far, the 21-year-old had a challenging night.
Although Mauricio Pochettino's team has scored goals in their last six games, they will need to do better in the second leg if the Argentine manager hopes to win his first trophy in England at the end of February.
Since the beginning of the previous season, no Premier League team has lost more games away from home than Chelsea (21)—only Bournemouth and Nottingham Forest (32 each) have lost more than Chelsea (31) in all competitions.
With a win, Boro—who has lost six times at home in the Championship this year—has a chance to go to the competition's final, which they won 20 years prior.