Chelsea missed a host of chances as Hayden Hackney's first-half goal handed Championship side Middlesbrough a slender advantage in Tuesday night's EFL Cup semi-final first leg at the Riverside Stadium.
Mauricio Pochettino's Blues must win at Stamford Bridge in the second leg on Tuesday 23 January or Boro will be in the Wembley final on Sunday 25 February.
How the match unfolded
It was a night when Chelsea misfired in front of goal, drawing a blank for the sixth time this season.
Cole Palmer had a good opportunity when he was gifted the ball by Jonny Howson on the half-hour mark, but the former Manchester City man was unable to capitalise and drove wide from 18 yards.
At the other end Hackney's effort flashed narrowly past the post before he sent the Riverside crowd wild on 37 minutes with what proved to be the winning goal, a composed side-footed finish from Isiah Jones's delivery.
Palmer had three further chances to level before the break, the best of which came in added time when goalkeeper Tom Glover spilled Enzo Fernandez's shot, but he fired the loose ball over with the goal gaping.
Chelsea started the second half as they had finished the first with Noni Madueke's 53rd-minute header forcing a smart save from Glover, while Axel Disasi, Moises Caicedo and Conor Gallagher all tried their luck from distance.
Pochettino made a double change on 66 minutes, bringing on Mykhailo Mudryk and Armando Broja for Madueke and Fernandez respectively.
Mudryk was denied by Glover, Raheem Sterling's curler dropped wide and a header from sub Alfie Gilchrist was deflected for a corner as Chelsea were unable to penetrate a well-organised Boro defence.
Pochettino's side will look for an instant response when they return to Premier League action on Saturday at home to Fulham. Victory for Chelsea would secure a league double over their local rivals having beaten them 2-0 at Craven Cottage earlier this season.
Wednesday night action
The EFL Cup semi-final first-leg action continues on Wednesday night with an all-Premier League tie between Liverpool and Fulham at Anfield.
When these two met at the quarter-final stage of the competition in 2000/01, Liverpool won 3-0 with goals from Michael Owen, Vladimir Smicer and Nick Barmby en route to eventually triumphing in the final.
