Marcos Alonso's late header dragged a lacklustre Chelsea to a 1-0 Europa League win at Slavia Prague.
The Spain full-back ghosted clear in the box to nod home Willian's teasing far-post cross, to hand the Blues the slender advantage midway through their quarter-final.
Chelsea came very close drawing their last-eight first leg tie at the Sinobo Stadium, the Blues struggling for fluency throughout a niggly night in Prague.
Replacement Ruben Loftus-Cheek's smart spin and break turned Slavia's defence at the death however, before Willian served up the perfect cross.
Alonso's third goal of the season and second in the Europa League proved just enough for Chelsea then, but the Stamford Bridge men will hardly be happy with their performance.
Chelsea will now expect to complete the job and move through to the semi-finals when hosting Slavia in the return leg next Thursday. .
The Blues were relieved to prevail on the pitch, in another night where racism hit the headlines.
Chelsea's security team identified and barred entry to the stadium for three supporters, who had taken part in a video on social media where Liverpool forward Mohamed Salah appeared to be racially abused.
Three other supporters in the video did not attempt to enter the Sinobo Stadium.
The best Chelsea could muster in a piecemeal first-half was Willian's curled effort that struck the crossbar.
Slavia captain Simon Deli wasted a fine early chance by nodding wide. Then Vladimir Coufal failed to connect fully with a skewed header of his own.
Miroslav Stoch stung Kepa's palms, but his strike was never likely to beat the Chelsea stopper.
If Chelsea's first-half was flat, the opening to the second was little short of tetchy.
The Blues' glowering looks and gesticulations escalated amid increasingly raucous home support.
Lukas Masopust turned last man Andreas Christensen only to slip, letting Chelsea hugely off the hook.
Not even an hour had elapsed when Sarri threw on talisman Eden Hazard in place of Pedro, the Stamford Bridge boss itching for an injection of impetus.
Antonio Rudiger saw his low effort rebuffed by Ondrej Kolar, and then Willian dragged wide from a promising position.
Ibrahim-Benjamin Traore raced clear for Slavia, only to see his fine long-range effort tipped round the post by Kepa.
N'Golo Kante and Ruben Loftus-Cheek were also sent into the fray as Chelsea chased a breakthrough.
Jorginho had the ball in the back of the net, but the goal was chalked off with Olivier Giroud offside.
A poor punch by Kepa gifted an opening to Jan Boril, but the Chelsea keeper conjured a solid stop to deny the full-back, immediately sparing his own blushes.
Just when Chelsea looked like labouring to the draw however, the visitors conjured the move of the match even capped with a smart finish.
Loftus-Cheek powered through midfield on the turn, before spreading out right to Willian.
The Brazilian steadied himself and whipped in a far-post cross, that Alonso nodded into the corner of the ne