Ten-man Chelsea frustrated Arsenal as the Premier League leaders were held to a 1-1 draw in a chaotic derby at Stamford Bridge.
The Gunners were aiming to move seven points clear of second-placed Manchester City but will instead head into December with a five-point cushion at the summit.
They received a huge boost when Moises Caicedo saw red in the 38th minute, but Trevoh Chalobah's header put Chelsea in front early in the second half.
Arsenal fought back with Mikel Merino nodding in from a sublime Bukayo Saka cross in the 59th minute, but they were unable to complete the turnaround late on.
The gap between Chelsea and the Gunners remains six points, with the Blues having moved back into third place, leapfrogging Aston Villa.
How the match unfolded
A scrappy opening saw three yellow cards dished out within the first 13 minutes, while Saka tested Robert Sanchez before Declan Rice thwarted Pedro Neto with a brilliant tackle at the other end.
Yet Chelsea's task was made harder when key midfielder Caicedo lunged in wildly on Merino, catching his shin, and after a VAR review, referee Anthony Taylor upgraded his yellow card to a red.
Defying their numerical disadvantage, though, Chelsea took the lead in the 48th minute, with Chalobah glancing a superb header home from Reece James’ excellent corner.
Mikel Arteta turned to Martin Odegaard and Noni Madueke for inspiration from the bench, but it was Saka who provided the spark Arsenal needed as he twisted Marc Cucurella one way and then the other before floating in a delivery for Merino to head in.
Sanchez denied Merino a winner in the closing stages, lunging low to his left to keep out a deflected shot from the edge of the box, with Chelsea's goalkeeper also doing well to beat substitute Viktor Gyokeres to a loose ball as the Blues claimed a deserved point.
Indiscipline comes back to bite Chelsea again
Chelsea have now received six red cards in all competitions this season, and if they are to mount a serious title challenge, Enzo Maresca needs to ensure his team keep their heads.
Referee Taylor set the tone early, handing out four yellow cards in the opening 27 minutes – although three of those were for Arsenal players.
However, Caicedo – who was clearly fired up and had been snapping into challenges – got his tackle on Merino all wrong, and once the official went to the monitor, the writing was on the wall for Chelsea’s instrumental midfielder.
The Blues had been the better team before that, with Enzo Fernandez particularly effective as he bombed forward from midfield – the Argentine was denied by a superb Jurrien Timber challenge before seeing a stinging drive parried by David Raya.
Yet with Caicedo off, Fernandez had to play a more disciplined role in the second half, and even though Chalobah’s clever header put the hosts ahead, Arsenal’s man advantage gave them the impetus to get on the front foot.
Chelsea by no means caved despite the loss of a key player, though Maresca must now shuffle his midfield pack for Wednesday’s trip to Leeds United.
Cole Palmer, an unused substitute here after returning from injury, may well get minutes at Elland Road.
Merino delivers again
Gyokeres returned from injury as he came on as a second-half substitute, but it goes to show how highly Arteta values Merino that it was the Spain international who stayed on, with Eberechi Eze – the Gunners' north London derby hero last week – making way.
Merino, by that point, had already nodded Arsenal level.
Sure, Saka's delightful skill and cross teed him up – the winger has now assisted four Premier League goals against Chelsea – but Merino had to be in the right place at the right time and keep his effort down.
Merino has now scored 10 goals for Arsenal in the league, with seven of them coming via his head, and he has proved himself as an excellent alternative to Gyokeres when it comes to leading the Gunners’ attack, just as he did when Arteta was short of options last season.
It is testament to Chelsea, though, that from then on, the Gunners did not manage a single attempt on target until the 88th minute, when Sanchez got down to Merino's drive.
Arteta will be frustrated not to come away with the win, but he will no doubt know a point is a solid enough result ahead of Arsenal hosting Brentford on Wednesday.
Club reports
Arsenal report | Chelsea report
What the managers said
Enzo Maresca: "I think we showed we are heading in the right direction. I always say the same. The performance has been fantastic. We were a much better team than them 11 v 11. With 10 players it is difficult but we dealt with it outstandingly.
"We tried to exploit the best solution, the best space. We knew the space was for Malo and Estevao on that side. Tonight we can be happy even if we didn't win.
Mikel Arteta: "Right from the beginning you could sense how much was at stake. Every duel and action was full gas, very short sequences of play. With the 10 men we expected a different game, we planned it, and we went through it in the second half. They got a free kick and they scored. Then it becomes a very tricky game to play. Overall I think that when you don't win with 10 men for 45 minutes you have to be disappointed. But you have to know the week we had with tough opponents."
"The spirit of the team is intact. We've had a massive week both mentally and physically and I think overall it's a positive week."
Next PL fixtures
Key facts
Chelsea are unbeaten in each of their last nine Premier League London derbies (W5 D4), their longest run under the same manager since their first nine derby matches of Jose Mourinho’s second spell in 2013/14.
Arsenal failed to win a Premier League match in which their opponents received a first half red card for the first time since August 2010 against Liverpool, a 1-1 draw with Joe Cole sent off for the Reds.
Only Arsenal (eight) have scored more goals via corners than Chelsea (seven) in the Premier League this season.
Chelsea have received four red cards in 13 league games this season, twice as many as any other team – it is the earliest into a Premier League campaign the Blues have had four players dismissed.