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 best Chelsea could muster in a piecemeal first-half was Willian's curled effort that struck the crossbar.
If Chelsea's first-half was flat, the opening to the second was a little short of tetchy.
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.