Arsenal's hopes of a top-four Premier League place this season were dealt a blow as they suffered a 3-0 loss to lower-level visitors against Crystal Palace on Monday.
Arsenal were put to a sword by the clinical hosts in the first half, with goals from forwards Jean-Philippe Mateta and Jordan Ayu giving Palace a 2-0 lead.
The Gunners improved after the break, with Emil Smith Rowe and Martin Odegaard squandering big chances to get their team back into the match. His waste proved costly as Wilfried Zaha ended the game with a 16-minute penalty from time.
Tottenham Hotspur's 5-0 home win over Newcastle United on Sunday means they are fourth, having played a game more ahead of their north London rivals on goal difference.
The scoreline at Selhurst Park did not flatter an impressive Palace, which continued to improve under former Arsenal captain Patrick Vieira, with the win moving them to ninth place.
"I'm really proud," Vieira told Sky Sports. "We defended well and took our chances. We scored runs in the crucial moments of the game. We needed a big team performance and we did that so we are really happy.
"It's a good moment for the club but we know we need to be consistent in our performance and results."
Arsenal were in good form in the Premier League ahead of their visit to London on Monday.
Arsenal coach Mikel Arteta told Sky Sports: "Congratulations to Palace on the game they played, but the way we competed, we made it impossible for ourselves."
"I apologize to my supporters. We didn't have the courage to be present or dominate the situation today, so I'm most annoyed.
"First we need to raise our hands and get the criticism we deserve. Then we accept it, look at ourselves in the mirror and look to the next game. It wasn't good enough."
Arsenal's 1-0 win at Aston Villa last time was their fifth consecutive league win - the first time they were on such a run since May 2015.
But they struggled from the start to find any rhythm, with Palace being a much better side in the early stages.
It took the home team 16 minutes to break the deadlock, as Joachim Anderson's head sent a free-kick across the box to propel Matta to his fourth league goal of the season.
Eight minutes later and centre-back Anderson again became the provider, he picked up a perfect pass to Ayve, who controlled well before finding the net for the first time since December.
Arsenal could not open any note in the first half, with Matta going close to adding a third for Palace before the interval.
Content with a two-goal lead, the hosts sat back and invited the pressure after the break. Smith Rowe should have done better from close range before Odegaard widened his effort.
Zaha then broke over the counter and was awarded a penalty - only Raheem Sterling (23) and Jamie Vardy (21) have been awarded more penalties in Premier League history than an Ivory Coast forward - which he converted strongly. did.
Arsenal substitute Eddie Neketia hits the crossbar but it would have been too little, too late.
Later, Arteta said that he had spoken about the importance of winning individual battles.
"We discussed it right before kick-off, the way you win here, when you compete the way you should," he said. "It means you win doubles, have the presence and composure on the pitch to manage certain situations.
"They're a really physical team and the pitch isn't the best to do that so you have to find a way to do other things that you don't normally need to do. We didn't manage to do that."
A second loss in their last three league games opened the door for Arsenal's top four rivals and Arteta still has to do to ensure a Champions League return for the first time since 2016-17.
Fifth-placed Arsenal are on 54 points with Spurs and three points ahead of West Ham United and Manchester United, although all their top four rivals have played more games.