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 net.
Felix fires hat-trick in Benfica win
Joao Felix scored a hat-trick as Benfica beat 10-man Eintracht Frankfurt 4-2 in their Europa League quarter-final first leg clash.
The 19-year-old striker delivered a memorable display in Lisbon as the Portuguese club took a commanding advantage into next week's return encounter.
He became the youngest player in Europa League history to net a treble, while Ruben Dias also scored for the hosts.
The visitors had defender Enda N'Dicka sent off by English referee Anthony Taylor for a foul after 21 minutes.
Luka Jovic had equalised Felix's opener, and Frankfurt did not fall completely away, with substitute Goncalo Paciencia's late goal giving them a glimmer of hope.
Benfica boss Bruno Lage left out top scorer and former Frankfurt striker Haris Seferovic, who had to be content with a place among the substitutes, while captain Andre Almeida was rested and replaced by Sebastien Corchia.
A groin strain sidelined Frankfurt striker Sebastien Haller, but defender N'Dicka and midfielder Sebastian Rode were recalled.
Referee Taylor was soon called into action, booking Jovic, who is on loan from Benfica, for a foul on Rafa Silva after just four minutes.
Frankfurt dominated the early stages, effortlessly keeping possession, but it all unravelled after 21 minutes as N'Dicka conceded a penalty and was red-carded.
He could have no complaints at Taylor's decision following his challenge on last man Gedson Fernandes, and Felix stepped up to beat Frankfurt goalkeeper Kevin Trapp and put Benfica ahead.
Ante Rebic then followed Jovic by picking up a booking, and Frankfurt knew they had to regroup quickly following the penalty setback and seeing N'Dicka sent off.
But they responded impressively, drawing level five minutes before the break after Benfica's Ljubomir Fejsa lost possession, Rode found Rebic and his cross was met by Jovic, who swept the ball home.
Frankfurt were level, though, for just four minutes, with Felix claiming his second - this time with a 25-yard shot - to leave Frankfurt trailing once more.
An eventful first half ended after Frankfurt had a goal disallowed. Filip Kostic netted from the edge of the penalty area, but it was ruled out for offside.
Benfica then took control of the tie, scoring twice in four minutes as Dias struck with a close-range header, then Felix completed his hat-trick by side-footing home Alejandro Grimaldo's cross before Paciencia completed the scoring.