Manchester United beat Manchester City 2-0 on Sunday to complete a league double over their local rivals for the first time since 2009-10.
Goals from Anthony Martial and Scott McTominay handed the Red Devils all three points and means leaders Liverpool are now just two wins away from being crowned champions.
Sunday's only other Premier League fixture saw Chelsea tighten their grip on fourth place with a 4-0 win over Everton at Stamford Bridge.
Mason Mount and Pedro scored in the first half to put the Blues in control before Willian and Olivier Giroud found the net after half-time to secure all three points.
Chelsea 4-0 Everton
Manchester United 2-0 Manchester City
Winner: Chelsea's Future
Chelsea manager Frank Lampard's decision to put his faith in the club's youngsters this season continues to pay off with Billy Gilmour and Mason Mount putting in stellar performances against Everton.
Gilmour made his full Premier League debut at Stamford Bridge and was involved in the Blues' first two goals in a man-of-the-match display.
Goal's Nizaar Kinsella highlighted just how impressive the 18-year-old was against Everton:
Mount also produced a superb display, opening the scoring with a great turn and finish for his sixth Premier League goal of the season:
Further goals from Pedro, Willian and Giroud gave the hosts a comfortable lead and allowed Lampard to hand Premier League debuts to 18-year-old academy products Armando Broja and Tino Anjorin.
Kinsella highlighted Lampard's commitment to youth:
The win sees Chelsea cement fourth place in the Premier League but also shows the club's future looks to be in good hands thanks to their raft of talented youngsters.
Manchester City goalkeeper Ederson was guilty of two poor errors against Manchester United as the Citizens were beaten 2-0 at Old Trafford.
The normally reliable Brazilian ought to have saved Martial's shot for the opening goal. The stopper got down to the forward's effort but allowed it to squirm under his body (UK video only):
A poor touch almost gifted Martial a second goal at the start of the second half before he managed to salvage the situation with a sliding challenge.
Yet worse was to come for Ederson deep into stoppage time when he gifted the Red Devils the second of the match, which sealed the win.
A poor throw out from the Brazilian fell straight to McTominay, who showed great technique to send a first-time shot from range into the back of the net:
It was an afternoon to forget for Ederson and one that saw Manchester City slip to their seventh defeat of a disappointing title defence.
Winner: Anthony Martial
Martial sent Manchester United on their way to victory over Manchester City with the opening goal at Old Trafford on Sunday.
The Frenchman showed some quick thinking to latch on to a free-kick from Bruno Fernandes and net his 11th Premier League goal of the season.
The strike also saw him notch a rare landmark:
Martial was a danger throughout the match against a Manchester City backline that continues to look vulnerable:
It's an important win for Ole Gunnar Solskjaer's side that extends their unbeaten run to 10 games in all competitions.
Loser: Carlo Ancelotti's Chelsea Return
Everton boss Carlo Ancelotti made his first return to Stamford Bridge since being sacked in 2011 and told reporters in the buildup it would be an "emotional moment."
The Italian could be forgiven for feeling emotional after seeing his side thumped by the Blues with a defeat that all but ended their slim hopes of securing European football for next season.
The Toffees put in arguably their worst display since Ancelotti was appointed manager in December 2019 and were no match for a lively Chelsea team.
Opta noted how it's the joint-heaviest league defeat of Ancelotti's managerial career:
The game also represented the first time Everton have failed to find the back of the net in 11 league matches under Ancelotti.
The 60-year-old told Sky Sports after the match that "everything went wrong" for his side:
Everton have been much-improved under Ancelotti but have hit a mini-slump. They have now gone three games without a win and face league leaders Liverpool next in the Premier League.