Kai Havertz and Christian Pulišić struck the goals in either half as the hosts put themselves in a strong position to progress.
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Kai Havertz and Christian Pulišić were on target as Chelsea overcame LOSC 2-0 in this UEFA Champions League round of 16 first leg.
8': Havertz heads Chelsea in front
63': Pulišić slides in hosts' second
Match in brief: Holders building momentum
Fresh from winning the FIFA Club World Cup a week ago, Chelsea started authoritatively and Havertz could have had a hat-trick inside eight minutes. The German striker, who fired his first chance over and was then denied by Leo Jardim, took his third opportunity with a downward header from Hakim Ziyech's corner.
Jardim then saved from Marcos Alonso before the visitors established a foothold in the game, though they only had Edouard Mendy's save from Benjamin André's 36th-minute effort to show for their improvement before the break.
Thomas Tuchel's men wrested control back after the interval and Zeki Çelik's block from Alonso could only temporarily stem the tide. The second goal finally arrived in the 63rd minute thanks to a wonderful driving run through the centre from N'Golo Kanté, who slipped in Pulišić to place his finish inside the far post.
LOSC avoided further damage, but Jocelyn Gourvennec will know that his side need to show more potency up front if they are to have any hope of turning the tide in the return on 16 March.
PlayStation® Player of the Match: N'Golo Kanté (Chelsea)
"Everywhere from start to finish, recovering the ball, winning one on ones and then setting up Pulišić for the second goal. An outstanding game played at high intensity."
UEFA Technical Observer panel
Andrew Haslam, Chelsea reporter
The home side looked like they were going to blow LOSC away in the opening moments, with Havertz twice unable to convert promising openings before heading in in the eighth minute, but Chelsea were unable to capitalise on that and gradually lost their way as the tempo dropped. Such is the quality of their attacking options, however, that they always carried more threat despite LOSC enjoying plenty of possession, and Pulišić's assured finish put them in charge of the tie.
Ian Holyman, LOSC Lille reporter
Going back over the Channel with a 1-0 defeat, LOSC would have been relatively happy. At 2-0 down, the French champions now have a monumental task to reach the quarter-finals for the very first time. Chelsea didn't create as many opportunities as their dominance merited, but LOSC were toothless with Jonathan David not given a sniff, never mind a sight of goal. Renato Sanches' performance was a positive, but they need more at home in the second leg.
Christian Pulišić, Chelsea midfielder: "It's always nice to get on the scoresheet. We're in a good position now but we know it's not going to be easy over there. We need to work on our control more. The game felt a bit too open at times, but you expect that against a good team."
Kai Havertz, Chelsea forward: "We knew it was a difficult game. They played very good football and the first half was very difficult for us. We have a lot of good strikers so it's good to have a change and not make it easy for our opponent. We didn't play our best. We could not get our usual aggressiveness, but we won 2-0."
Thomas Tuchel, Chelsea coach: "We have another clean sheet and a well-deserved one. It was hard not to allow chances as they were a strong team. It was a bit up and down, but we never gave big chances away and were solid."
Jonathan David, LOSC forward: "They're the reigning Champions League winners, a quality team, and we knew it would be difficult. They scored when they had chances. We lacked a little quality in the final ball and that cost us. When you make mistakes at the top level, you pay for it dearly. That's what happened."
Joe Cole, BT Sport
"The only downside for Chelsea were the injuries to Kovačić and Ziyech. The second half was a very strong performance and Tuchel will be happy."
• Chelsea have won their last six home Champions League games, all without conceding a goal (F15 A0).
• Edouard Mendy has now kept 14 clean sheets in 18 Champions League appearances – nine in 12 matches last season and five in six games this term.
• The Blues' results in all competitions when Havertz has scored for them is W9 D4 L0.
• Chelsea have conceded just one goal in their last nine Champions League matches at Stamford Bridge.
• LOSC have lost their last five away games against English opposition in UEFA club competition.
N'Golo Kanté – 11
Thiago Silva – 10
Antonio Rüdiger – 8
Kai Havertz – 8
Christian Pulišić – 8
Chelsea: Mendy; Christensen, Thiago Silva, Rüdiger; Azpilicueta, Kanté, Kovačić (Loftus-Cheek 51), Alonso (Sarr 80); Ziyech (Saúl Ñíguez 60), Havertz, Pulišić (Werner 80)
LOSC: Jardim; Zeki Çelik, Fonte, Botman, Tiago Djaló (Gudmundsson 76); Onana (Burak Yılmaz 65), Xeka, André; Renato Sanches (Ben Arfa 81), David (Zhegrova 81), Bamba