Marcos Alonso's late header handed Chelsea a narrow 1-0 victory in their Europa League quarter-final first-leg with Slavia Prague.
Alonso ghosted in to meet Willian's inch-perfect ball with four minutes of normal time remaining to give Chelsea a scarcely deserved win in the Czech Republic.
It was just their second attempt on target in a below par performance and for long periods it looked like Willian's bending effort which crashed against the bar would be the closest they came to a goal.
But the Brazilian was their best player on the night and his moment of quality means they'll head back to Stamford Bridge for the second leg with a slender advantage.
Chelsea: Kepa (6), Azpilicueta (6), Rudiger (6), Christensen (6), Alonso (6), Jorginho (5), Barkley (6), Kovacic (5), Pedro (5), Giroud (5), Willian (7).
Subs: Hazard (6), Kante (6), Loftus-Cheek (n/a).
Slavia Prague: Kolar (6), Coufal (5), Ngadeu-Ngadjui (6), Deli (6), Boril (7), Traore (6), Kral (6), Masopust (7), Sevcik (6), Stoch (6), Olayinka (6)
Subs: Zmrhal (6), Husbauer (6), Van Buren (n/a)
Slavia started quickly and had the first chance after two minutes - skipper Simon Deli could not direct his header on target from a wicked Miroslav Stoch ball into the back post.
The game was a turgid affair and Chelsea flattered to deceive for much of the half. They failed to muster a single shot on target, although Willian did strike the bar after cutting onto his right foot on the edge of the box.
They came out with renewed vigour after the break and began to look like the superior side. Olivier Giroud shot wide after a well-worked short corner, before Antonio Rudiger fired in a low volley that was cleared by Kolar's feet.
Sarri made seven changes to the side who beat West Ham on Monday night. Luiz, Emerson, Kante, Loftus-Cheek, Hudson-Odoi, Higuain and Hazard all dropped out, with Christensen, Alonso, Barkley, Kovacic, Pedro, Willian and Giroud coming into the side.
Eden Hazard was sent on with 30 minutes remaining and Willian almost gave Chelsea the lead immediately, dragging a left-foot shot wide from a tight angle.
Ibrahim Traore then tested Kepa with a well-struck drive from the edge of the area to remind Chelsea of the quality they possessed on the break and the Spaniard had to be alert to push Josef Husbauer's volley away.
But Chelsea struck late to take an advantage with them back to London. Willian looked to have few options on as he received the ball midway through Slavia's half. He looked up and clipped in a fantastic cross for Alonso, who showed good composure to nod the ball into the bottom corner.
Man of the match - Willian
The Brazilian looked the player most likely to break the deadlock for Chelsea and although he wasn't the match-winner, it was his cross that set up Alonso for the winner.
He had the beating off both of Slavia's full-backs, hit the bar, nearly squeezed an effort in from a tight angle and gave Sarri a headache in wide positions ahead of Sunday's trip to Liverpool.
Chelsea are live on Super Sunday as they look to put a huge dent in Liverpool's title challenge at Anfield. Kick-off is at 4.30pm.