Arsenal is in real danger of being eliminated from the last 16 of the Europa League after Nacho Monreal's own goal and a late Ismaila Sarr header in the second half secured Rennes a deserved 3-1 win at Roazhon Park.
The Gunners looked to be heading back to London with a serviceable 2-1 defeat after having Sokratis Papastathopoulos sent off in the first half, but Sarr's dramatic late goal put daylight between the sides to leave Arsenal seemingly on the brink.
It all started so well for Unai Emery's men when Alex Iwobi's somewhat fortuitous early effort gave them an away goal that may yet prove decisive.
The good feeling did not last as Sokratis was given his marching orders for two yellow cards, and Benjamin Bourigeaud netted a stunning equaliser as a result of the second offence.
The home side looked desperate to make the most of its numerical advantage and the pressure paid off, as Monreal deflected a cross into the goal and Sarr rounded off a brilliant performance with an emphatic diving header.
It took Arsenal just three minutes to break the deadlock, as Iwobi cut into the left side of the area and saw an apparent low cross curl in off the right-hand post.
But Rennes was back in it late in the half.
Sokratis cynically hauled Sarr down when seemingly through on goal to earn a second booking in the space of seven minutes, and Bourigeaud brilliantly found the top-right corner with a stinging half-volley when the subsequent free-kick came back off the wall.
Rennes stepped things up after the break and crafted a string of chances – Petr Cech denying goal-bound efforts from Bourigeaud and Sarr.
Cech was helpless in the 65th minute, however, as Mehdi Zeffane's cross took a wicked touch off Monreal and squeezed inside the near post.
Sarr finished Arsenal off two minutes from time, meeting substitute James Lea Siliki's sublime cross at the back post with a fine diving header.
What does it mean? Arsenal facing early exit
On the face of it, winning 2-0 at home is certainly not beyond Arsenal, as that scoreline would be enough to send it through. But, even before Sokratis' red card, Rennes showed plenty of attacking threat and is not a side to sit back and absorb pressure. The fact Arsenal has to come out and attack it next week could just play into its hands, given the pace and dynamism the Ligue 1 side has in the final third.
Ismaila a Sarr in the making
Cech enjoyed arguably his best performance in an Arsenal shirt, but the standout player was still Sarr. The 20-year-old winger tormented the Gunners' left flank tirelessly and ultimately got the goal that transformed the tie.
Hapless Sokratis leaves Arsenal in trouble
Although Sokratis might feel his first yellow card was harsh, he could have no arguments about the second as he foolishly got caught out by Sarr. The referee had no option but to dismiss him and Arsenal never recovered.
Rennes hosts Caen in Ligue 1 on Sunday before next week's return leg, while Arsenal faces Manchester United the same day at Emirates Stadium in a match likely to be pivotal for the two teams' top-four aspirations.