Arsenal got the better of their London rivals with a 1-0 victory over the Blues.
The game was decided by a set piece - Mikel Merino glancing a header into the net from a corner, ending the Gunners' long goal drought from the particular set piece.
Chelsea's attack never got firing as they struggled without Cole Palmer, who missed the game through injury. Marc Cucurella went closest with a volleyed effort, spilling out of the grasp of David Raya and narrowly avoiding the target.
The Blues remain in the top four as it stands, but could drop to fifth if Newcastle win their game in hand.
Here are four things we learnt from the match:
Many Chelsea fans will be eager to learn why Enzo Maresca prefers to play Reece James in a midfield role, rather than his usual spot at right back.
This is largely where the Chelsea captain has been played since his return to the team, with Maresca opting to play a centre back in his place - usually Trevoh Chalobah or more recently, Wesley Fofana.
He looked lost in the first half, unable to mark Arsenal's midfield effectively and allowing the Gunners to progress the ball with ease through the middle.
James was named in Thomas Tuchel's first international squad and will re-unite with the German for the first time since he left the club in 2022.
Tuchel sees him as a energetic, attacking full-back. Maresca, seemingly, does not.
Enzo not suited to attacking midfield role
Cole Palmer was a rare absentee from the travelling squad, and it was later revealed he had picked up a 'small' injury in yesterday's training session, being deemed not fit enough to feature - even from the bench.
With James occupying the midfield alongside Moises Caicedo, Enzo Fernandez was moved into the number ten position, and looked far from his recent best.
Chelsea's front four of Christopher Nkunku, Pedro Neto and Jadon Sancho, alongside Fernandez, managed zero touches in Arsenal's box in the opening half hour. Their first shot on target came via Marc Cucurella; the left back looked the most likely to score in the game.
Of course, injuries have restricted Maresca from fielding his best players in their best positions, but given Kiernan Dewsbury-Hall and Nkunku's availability for this game and history in attacking midfield, questions will be asked.
The Sanchez situation
Maresca was confident enough to confirm Robert Sanchez's re-inclusion to the Chelsea first team - but the shot-stopper looked particularly nervy at the Emirates, and was the target of playful teasing from the home fans.
His decisions to dive for loose balls put his teammates under real pressure, and he looked less from convincing after some pinball clearances in the Chelsea box.
Arsenal's decision to flood the keeper resulted in their goal, but Sanchez did grow as the match went on - notably making a clearance off the line to prevent his side from conceding further.
Maresca is expected to stick with the Spaniard until at least the end of the season, but the club's sporting directors may be putting real thought into signing a new number one in the summer.
A change in fortune after the international break?
Chelsea have won four from their last five in all competitions, but after matches against Leicester and Southhampton, as well as a two-legged tie against Copenhagen, Arsenal proved to be the biggest test - and they were able to play through Maresca's side fairly easilly.
A gap in the season may come as an opportune time for the club, as Maresca will have time to tweak his system and find ways for his side to improve.
Furthermore, he will hope to have Nicolas Jackson and Noni Madueke back after the break, alongside Palmer. The trio have racked up over half of Chelsea's total league goals this season, with thirty between them.
Wesley Fofana and Romeo Lavia have already returned to the first team following a lengthy period on the sidelines, and will help to shore up the defence.
Chelsea will cling on to their top five position in the league, but face tougher tests on the horizon, and are still targeting European success in the Conference League.
A squad nearing full fitness will be a huge boost for Maresca as he looks to deliver on his goals.