Paris Saint-Germain 2-1 Chelsea
John Obi Mikel scored a vital away goal for Chelsea as Paris St Germain inflicted the first defeat of Guus Hiddink's second spell in charge at Parc des Princes.
Mikel made amends for conceding a free-kick from which Zlatan Ibrahimovic scored to put PSG 1-0 up in the Champions League last-16 first leg.
The striker's shot deflected off Mikel, who equalised from close range for a rare goal, before substitute Edinson Cavani struck to earn PSG a 2-1 win and end Hiddink's 11-match unbeaten run.
It was Chelsea's first loss since the December defeat at Leicester which marked the end of Jose Mourinho's tenure.
Laurent Blanc's side have lost once in 33 games this season, while Hiddink has two defeats in 34 games across two spells as Chelsea caretaker boss.
The Dutchman will be content, though as Mikel's first Chelsea goal since December 2014 means the Blues have a vital away goal ahead of the 9 March return at Stamford Bridge.
Away goals have separated these sides in the Champions League in each of the last two seasons.
Chelsea might have had theirs sooner had it not been for a fine reflex save from Kevin Trapp.
Trapp stuck out a hand to save a Diego Costa header which had looked destined for the back of the net and ended up hitting the bar.
Chelsea performed well, although Eden Hazard, who chose the eve of the match to reveal it would be difficult to turn down a move to PSG, made a limited impact.
The Belgian playmaker did little to boost any potential transfer fee and was withdrawn 19 minutes from the end.
John Terry missed the trip to Paris and may have to wait for his 700th Chelsea appearance as he is a doubt for Sunday's FA Cup fifth round tie with Manchester City.
Gary Cahill and Branislav Ivanovic performed admirably in the absence of Terry and Kurt Zouma, who is out for the season with a knee injury.
Baba Rahman came into a reshuffled defence, which survived early pressure before the left-back delivered a cross which the masked Costa met, only for Trapp to somehow turn the ball on to the bar. It felt like a chance Chelsea had to take.
Lucas Moura had impressed early on and almost provided the opener for Ibrahimovic - but the Swede headed the ball into the ground.
Moura burst by three Chelsea players before he was tripped by Mikel, who was booked. And Ibrahimovic's free-kick hit the midfielder and spun past the wrong-footed Thibaut Courtois and into the net.
Six minutes remained until half-time and Chelsea replied when Costa met Willian's corner ahead of Ibrahimovic, getting enough of a flick-on for the ball to drop for Mikel. He finished emphatically from six yards for his sixth goal in 358 Chelsea games.
Trapp raced off his line to deny Costa early in the second half before Courtois repelled PSG time and again - only for his defences to eventually be breached for a second time.
First Courtois tipped Angel di Maria's effort over, then he blocked an Ibrahimovic shot and next he saved from Moura.
Chelsea were defending deep, inviting pressure. Courtois denied Di Maria from a free-kick before the former Manchester United winger played a fine ball over the top for Cavani, who stayed onside before tucking the ball under the goalkeeper from a tight angle.
Chelsea's interim boss Guus Hiddink told BT Sport Europe: "Of course the result is negative on one side but on the other hand it's also important to have an away goal.
"But it hurt a bit at the end because I think the team was playing very decently against a team which has a lot of capacity to play.
"So at the end, yeah it hurt a little bit, but still we are in a race. When there are two legs then it's important to have the away goal. At the end, if we can do it at Stamford Bridge then the defeat doesn't count so much."
Defender Cahill says Chelsea now know what they have to do in the second leg. He said: "We pushed for the goal tonight, managed to get it. Disappointed at the end. 1-1 here would be a fantastic result. They're a fantastic team and that was very, very tough.
"It's disappointing at the end because I thought we defended throughout the game really well and we dug in.
"It'll be a different game at home, but nevertheless we go for the home leg.
"We know what we have to do now and we're more confident at home. We know how tough it is to come here; you see their record this season, not just in the league, but in European competition as well. It's very, very tough to get a result here.
"To concede two, it's hard when they have so many corners and final-third entries. It's very tough. But it's poised now. We know what we have to do and we have done it before at home. We go all in now."