Misfiring Arsenal spurned the opportunity to reclaim first place in the Premier League as they lost 2-0 to spirited fellow Londoners West Ham United at the Emirates on Thursday.
West Ham only had three efforts on target but Tomas Soucek gave them a halftime lead before former Arsenal defender Konstantinos Mavropanos extended the advantage after the break.
Arsenal had more than twice as many chances but failed to take any as the visitors defended gamely.
Second-placed Arsenal remain on 40 points, two behind Liverpool who beat Burnley 2-0 on Tuesday.
"This is a very disappointing night for us," Arsenal manager Mikel Arteta told reporters. "We have to put the ball in (the net) and win it (the game)."
West Ham took the lead against the run of play after 13 minutes when Soucek netted after a pull-back from Jarrod Bowen who looked as if he just managed to keep the ball in play.
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"It's difficult because the ball was in the air," Bowen told Amazon Prime. "It was hard to tell. It would have been easier to tell if it had been on the ground."
After that it was an Arsenal blitz with Bukayo Saka hitting the post and Gabriel Jesus missing two good chances before Mavropanos scored his first league goal for the Hammers with a thumping header from a James Ward-Prowse corner in the 55th.
"I think the best moment to score my first goal for West Ham and in the Premier League," said Mavropanos. "We work a lot on the set-pieces, so it was great. It is an important win for everyone and for our boss. We are really happy."
West Ham could have made it three from three chances but Said Benrahma had a penalty saved by David Raya in added time after a foul by former West Ham captain Declan Rice.
The defeat raises familiar questions about Arsenal's title credentials. They led the league this time last year only to falter in the run in and be overtaken by Manchester City.
It may have been one of those nights when things just did not happen in front of goal but finishing remains an issue.
As well as striking the post, Saka also had a near point-blank header stopped by Alphonse Areola, who made a string of fine saves in the second half too.
Jesus should have a scored when Arsenal were still one goal behind but headed over an open goal while fellow Brazilian Gabriel Martinelli also missed good opportunities.
The win leaves West Ham in sixth place on 33 points and marked a landmark night for their manager David Moyes who had never won at Arsenal in the league in 22 attempts.
