Marcos Alonso's late goal secured a 1-0 win from a difficult match in Prague.

Sarri said his side have improved from earlier in the season, when they would "immediately concede" in difficult moments.

"In the past we were not able to suffer in the difficult moments of a match," said the Italian.

"Now we are able to stay in trouble and suffer without conceding."

An under-strength Chelsea struggled to find any rhythm for large spells, but grew into the game after Eden Hazard was introduced off the bench just before the hour mark.

Alonso timed his run perfectly to meet Willian's right-wing cross with a firm header four minutes from time at the Eden Arena.

The second leg at Stamford Bridge takes place on 18 April.

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Late goal papers over performance

Chelsea return home with a lead, but this was definitely not a vintage European performance from the 2013 Europa League winners.

They were sloppy in possession from the first whistle as Sarri made seven changes from Monday's dominant victory over West Ham in the Premier League.

Willian hit the crossbar with a fierce shot in the first half, but it took until the 59th minute for Antonio Rudiger to register their first shot on target, moments after Hazard came on.

The Belgian was influential as Chelsea played with more intensity and posed more of a threat, while Willian shot narrowly wide after a neat one-two with Olivier Giroud.

Left-back Alonso had a poor game by his high standards, but popped up with a late run into the area to stun the home crowd.

Chelsea have never lost a European knockout tie having won the first leg away from home, and they travel to Premier League leaders Liverpool on Sunday hoping to make it five consecutive wins in all competitions.

Slavia can pose a threat at the Bridge

Czech league leaders Slavia have scored 10 goals in the knockout stages, and showed enough quality against Chelsea to suggest this tie is not over.

Captain Simon Deli almost handed the hosts an early lead when he headed Miroslav Stoch's free-kick inches wide at the back post.

Former Blues midfielder Stoch was bright throughout as he pulled the strings for Slavia, who had more shots in total and more on target as they played with an intensity that Chelsea struggled to cope with in the early exchanges.

They almost snatched a goal moments before Alonso's winner when Ibrahim Benjamin Traore produced a fierce shot at goal, but Kepa Arrizabalaga leapt to his right to save well.

Jindrich Trpisovsky's side forced Chelsea into mistakes and they will draw confidence from their impressive performances on the road against Genk and Sevilla in the previous rounds.

Man of the match - Antonio Rudiger (Chelsea)

Antonio Rudiger made more passes than any other Chelsea player (81) and was impressive as they kept an eighth clean sheet in this seasons's Europa League

English dominance at the back - the stats

• Slavia Prague remain without a win against English sides in European competition since a 2-1 victory against Leeds United in the quarter finals of the 1999-2000 Uefa Cup (P8 W0 D2 L6 since).
• Chelsea have lost only one of their past 15 European games (W11 D3 L1) and are unbeaten since a 3-0 defeat in the last 16 of the Champions League against Barcelona last season.
• No side has kept more clean sheets in the Europa League this season than Chelsea (8 - level with Arsenal).
• Chelsea had two shots on target in the match - the fewest they have managed in a Europa League game this season.
• Since his Chelsea debut in September 2016, Chelsea's Marcos Alonso has scored more goals in all competitions than any other defender (17).
• Willian has been directly involved in seven goals in his past six Europa League games for Chelsea (2 goals, 5 assists).