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 failed to score for the sixth time this season.
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.