Chelsea and Napoli closed in on the Europa League quarterfinals, but Arsenal face a major battle to reach the last eight after a 3-1 defeat away to Rennes on Thursday.
Alex Iwobi gave the Gunners a perfect start in north-western France at a raucous Roazhon Park, but the match hinged on the sending-off of Arsenal defender Sokratis Papastathopoulos four minutes before half-time.
Benjamin Bourigeaud equalised moments later, and the hosts' second-half pressure paid off as a Nacho Monreal own goal put Rennes ahead before Ismaila Sarr's late header gave them a two-goal cushion in the tie.
Unai Emery's side must now turn the tie around in the second leg in London next Thursday if they are to progress any further in a competition which represents their only hope of silverware this season, and offers a potential route into next season's Champions League with Arsenal sitting outside the top four in the Premier League.
After Iwobi's early opener, the game turned just before half-time when Hatem Ben Arfa played in Sarr, who was hauled back by Sokratis just outside the area. The Greek defender had already been booked, so a second caution equalled a red card.
Bourigeaud struck the wall from the resulting free-kick, but the ball came back to him and he sent a stunning half-volley into the far corner.
Chelsea may also need this competition to get back in the Champions League next season and the Blues eased past Dynamo Kiev 3-0 at Stamford Bridge courtesy of goals from Pedro, Willian and Hudson-Odoi.
Pedro could have had at least a hattrick as Denys Boyko denied him four times after the Spaniard had turned home Olivier Giroud's flick to give Chelsea an early lead.
Willian was more accurate with a brilliant free-kick 25 minutes from time to double Chelsea's lead and substitute Hudson-Odoi surely put the tie beyond Kiev with a smart finish a minute from time.
"The third goal for us is really very important," said Chelsea manager Maurizio Sarri.
"3-0 is an important result, but of course we have to play the second leg so we have to be careful."
Sarri's former side Napoli are also well on course for the quarters as early goals from Milik and Fabian Ruiz, plus a second-half own goal from Jerome Onguene, earned a 3-0 win over last season's semifinalists Salzburg.