Arsene Wenger finally recorded a Premier League victory against Jose Mourinho as Arsenal kept their slim top-four hopes alive with a well-deserved win over Manchester United.
The under-fire Frenchman had failed to beat Mourinho in the pair's previous 12 league meetings but he got his first three points against the former Chelsea boss just when he needed it.
Granit Xhaka's deflected strike gave the hosts a lead at the start of the second half before ex-United striker Danny Welbeck scored his third in as many games against his former employers to secure a 2-0 win.
Arsenal remain sixth despite the win but will still harbour hopes of closing an eight-point gap on Manchester City in fourth.
United still had a strong side despite making eight changes from their Europa League semi-final first leg against Celta Vigo but Mourinho's unblemished record against Wenger never looked like remaining intact.
The likes of Phil Jones, Chris Smalling, Juan Mata and Wayne Rooney came into the side for the visitors, with the latter guilty of missing a great first-half chance.
The recalled Rooney, who has scored more Premier League goals against the Gunners than any other player, headed over an early Mata cross.
Arsenal tested David De Gea soon after but the Spaniard easily saved an Alexis Sanchez effort.
The lively early start continued as Anthony Martial ghosted in behind the three-man Arsenal defence to collect a Rooney pass only to see his shot turned behind by Petr Cech.
Both goalkeepers were called on to keep their sides level and it was De Gea who showed why he had been named in the PFA Premier League team of the year as he dived low to tip an Aaron Ramsey shot away from goal following a superb Mesut Ozil pass.
The chances dried up slightly but the mood had been set, Welbeck came close as the half wore on, fit-again Jones bravely blocking his close-range shot.
Smalling was the next to prevent his former colleague opening the scoring with a similar effort as the hosts began to ramp up the pressure.
Alex Oxlade-Chamberlain sold Rooney a dummy and then forced another save out of De Gea with a long-range strike.
Their good work was almost undone as Rob Holding played a pass into his own box which Rooney was alive to, beating Laurent Koscielny to the ball but not tucking it past Cech when clean through.
The half petered out but Arsenal needed less than 10 minutes after the interval to take the lead - albeit with a huge slice of luck.
Xhaka lined up a long-range strike and his speculative effort took a big deflection off the back of Herrera to loop up over De Gea to open the scoring.
And the hosts needed a little over three minutes to double their advantage as Welbeck rose unmarked to head home an inch-perfect Oxlade-Chamberlain cross.
Mourinho turned to Jesse Lingard and Marcus Rashford from his bench but it was Rooney again testing Cech with a low free-kick.
Arsenal had United at arm's length for the remainder of the contest, seeing out the game with professionalism and giving their manager a victory to savour in a forgettable season.
Mourinho selected an under-strength side and United were floored by two goals in quick succession early in the second half. Xhaka opened the scoring and former United forward Welbeck gleefully added the second goal with a powerful header.
A new club for Mourinho but still Wenger could not take three points from the Red Devils' boss in their first meeting at Old Trafford. Juan Mata had put United ahead just after the hour mark but Arsenal fought back to earn a point as Olivier Giroud came off the bench to head home an 89th-minute equaliser.
This was just a second league win of the season for Mourinho's struggling champions as Kurt Zouma put the Blues ahead before Gabriel was sent off for the visitors before the break. Santi Cazorla's dismissal saw Arsenal go down to nine men, with a last-minute Calum Chambers own goal sealing the points for Chelsea.
A battling draw for champions-elect Chelsea as they brought an end to a fine run of wins for their hosts. Wenger's side created far more chances but the point took Mourinho's men 10 points clear of Manchester City at the summit.
Wenger and Mourinho clashed on the touchline as Eden Hazard put Chelsea ahead from the penalty spot. Diego Costa's ninth goal in seven games following his arrival from Atletico Madrid wrapped up the points and put Chelsea five points clear at the top of the table.
Wenger's 1000th game in charge of Arsenal was one to be remembered for all the wrong reasons for the long-serving Frenchman. Early goals from Samuel Eto'o and Andre Schurrle put Chelsea in charge before Kieran Gibbs was incorrectly sent off for an Alex Oxlade-Chamberlain handball which saw Hazard score from the spot. A brace from Oscar and a strike from Mohamed Salah secured Mourinho's biggest-ever win as Blues boss.
An instantly forgettable stalemate, which meant Arsenal missed the chance to top the table heading into Christmas. Frank Lampard hit the bar for Chelsea while Giroud missed two decent chances to take the points for the hosts.
Wenger may not have managed to break his duck against Mourinho - but this draw at the Emirates Stadium saw Manchester United crowned champions at the expense of Chelsea. Gilberto Silva's penalty put Arsenal ahead after Khalid Boulahrouz had been sent off for fouling Julio Baptista, with Michael Essien pulling a goal back for the Blues.
Mathieu Flamini's strike 12 minutes from time looked to have Arsenal on course for a rare Premier League win over Chelsea. But Essien levelled before both he and Lampard hit the woodwork in stoppage time as Mourinho's side pushed on for the victory.
Chelsea moved nine points clear at the head of the table courtesy of a well-deserved win at Highbury. Arjen Robben scored before the interval before Joe Cole wrapped up the win after Thierry Henry had hit a post and Robin van Persie had a goal disallowed for offside when the scores were level.
Didier Drogba's lucky goal secured a first Premier League win for Chelsea over their London rivals in almost a decade. The Ivory Coast striker tried to control a cross but saw the ball hit his knee and fly past Jens Lehmann to ruin Wenger's 500th game at the Arsenal helm.
A win for Chelsea would have seen them win their first title in 50 years and wrestle the crown from the reigning champions. Instead the pair played out a goalless draw in which Robert Pires hit the bar and missed a great chance, while Drogba and Lampard threatened for the home side.
The first Premier League meeting of Wenger and Mourinho saw both settle for the share of the spoils following an enthralling draw at Highbury. Thierry Henry opened the scoring in sensational fashion before putting the Gunners back in front with a quickly-taken free-kick after John Terry had equalised. Eidur Gudjohnsen scored the second leveller of the day for Mourinho's men, with Flamini seeing a late penalty appeal rejected.