Arsenal's Premier League title hopes suffered another major blow as they went down to a 2-1 home defeat against struggling Swansea City on Wednesday.
Joel Campbell put Arsene Wenger's men into an early lead, but Wayne Routledge's first top-flight goal since December 2014 had the visitors level.
The Gunners hit the woodwork twice in the first half through Alexis Sanchez and Olivier Giroud, with the Chile international doing so again after the break, a few moments before Ashley Williams' scruffy close-range winner stunned the Emirates.
Arsenal were heavily criticised for their defeat at Manchester United on Sunday and have now only won two of their last eight league fixtures at a crucial stage of the campaign.
Wenger's side remain six points behind leaders Leicester City with 10 matches remaining. Swansea, who were without head coach Francesco Guidolin due to a chest infection that meant he was taken to hospital earlier in the day and made six changes to their team, stay 16th, but significantly open up a welcome six-point cushion on the bottom three.
There were three alterations for the hosts, with Per Mertesacker replacing the injured Laurent Koscielny, while Campbell and Giroud came in for Theo Walcott and Danny Welbeck.
Arsenal almost moved in front when Mesut Ozil headed on to Sanchez, with the attacker mis-hitting his original effort before composing himself and striking a right-footed shot against the post as Lukasz Fabianski gratefully collected the rebound.
The Gunners did find the opener after 15 minutes, though. Sanchez cleverly sent a chip into the box and Campbell, on the slide, hooked an impressive left-footed finish into the far corner.
But after a slow start, Swansea came back into the game and were level in the 32nd minute. Arsenal wanted a free-kick on the halfway line when Jordi Amat challenged Ozil, but referee Robert Madley was unmoved.
The visitors then broke quickly and Jack Cork produced an excellent throughball, allowing Routledge – one of the six brought into the team - to take one touch and confidently slot past Petr Cech.
Arsenal should have regained the lead shortly before half-time, but Giroud could only smash his right-footed volley against the crossbar from 10 yards after Mertesacker had perfectly headed an Aaron Ramsey cross into his path.
The home fans were unhappy to see Wenger bring off Campbell when he opted to introduce Welbeck for the final 26 minutes and they had their hearts in their mouths a few moments later when substitute Gylfi Sigurdsson surged through on goal, rounded Cech but fired wide from a tight angle.
Having survived that scare, Arsenal hit the woodwork for a third time after Ozil had been fouled by Williams. Sanchez took the free-kick from 20 yards out and curled an excellent effort against the bar.
It looked like the match was set up for a late charge from Wenger's men, but it was Swansea who moved in front 16 minutes from time when Sigurdsson sent in a marvellous set-piece from the right and Williams beat Cech to the ball before bundling home from close range.
Walcott replaced Sanchez for the closing stages and Ramsey had a penalty claim turned down after he was challenged by Routledge, but the visitors held on for a shock first victory in five league games.
Arsenal, meanwhile, lost back-to-back league games for the first time this season and suffered a third defeat in a row in all competitions following a Champions League setback at the hands of Barcelona.
