Arsenal missed out on a chance to reclaim the Premier League lead on Thursday, falling 2-0 to West Ham, while Tottenham were defeated in a six-goal thriller at Brighton.
For the second year in a row, Mikel Arteta's Gunners were top of the tree at Christmas, but they trail leaders Liverpool by two points at the halfway point of the season.
Last year's runners-up dominated possession but lacked a cutting edge against the disciplined visitors, who Tomas Soucek and former Arsenal player Konstantinos Mavropanos scored.
Soucek scored from close range in the 13th minute after Jarrod Bowen's pass, with the goal confirmed after a lengthy VAR check to determine whether the ball had gone out of play.
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Arsenal pushed forward in waves, and West Ham goalkeeper Alphonse Areola saved a Bukayo Saka header on the half-hour mark.
As the clock approached halftime, the England winger hit the post from a tight angle, but Arsenal trailed at the break despite having nearly three-quarters of the possession.
The Gunners were determined to respond, but Mavropanos' bullet header cannoned in off the crossbar from James Ward-Prowse's corner, putting them 2-0 down.
Gabriel Jesus failed to beat Areola with a well-placed header and then missed another golden opportunity with his head at the back post.
Arsenal continued to bombard the visitors' goal but could not break through, and things could have been worse had Said Benrahma converted a late penalty.
The defeat drops Arsenal to second place on 40 points, one point ahead of Aston Villa and three points ahead of Manchester City, who have a game in hand.
Arteta stated that his team deserved more recognition for their dominance.
"We want to be at the top and today we are very, very disappointed," he told the BBC.
"We had so much dominance in the game. We are in a really good position. Today is a disappointing night. We have to pick ourselves up for a big game against Fulham."
He added: "The players put an enormous effort in. It's a difficult dressing room right now but we have another game in 72 hours. This is football, we have to improve in certain areas."
West Ham manager David Moyes celebrated his team's rise to sixth place at the table.
"We are not one of the top teams," he told Amazon Prime. "We are trying to get up and grow the team. For West Ham to be in Europe (they topped their Europa League group) is a great period."
