Chelsea suffered a fourth straight league defeat, falling to rivals Manchester United by a narrow 1-0 scoreline.
Bruno Fernandes set up Matheus Cunha in the first half for the only goal of the game, taking advantage of a temporary player advantage and punishing Chelsea's sloppy defending.
The Blues picked up the intensity after the break and hit the woodwork twice as they pushed for a goal, but had no luck as United held on to their three points.
The win gives Michael Carrick's side an increased buffer as they move ten points clear of the sixth placed Blues. As for Chelsea, they stay level on points with in-form Brentford and Bournemouth, and could potentially fall to eighth by the end of the weekend.
Story of the match
Manchester United had only won once in their last twelve visits to Stamford Bridge, and were looking to bounce back in the league after a disappointing defeat to relegation-battlers Leeds United.
Injuries and suspensions left Michael Carrick with only one natural centre-back in his starting lineup - 19-year-old Ayden Heaven, who has not yet started a game under the interim head coach.
As for Chelsea, Liam Rosenior was looking to end a three match losing streak in the league, but would need to do so without star striker Joao Pedro, who was missing from the squad due to injury.
Kick-off was preceded by a fan protest attended by fans of Chelsea as well as RC Strasbourg, against their ownership group BlueCo. Chelsea posted record financial losses this month - perhaps a sign that the consortium's unorthodox methods are not working out.
Enzo Fernandez made a return to the team following his disciplinary suspension by the club, though his midfield partner Moises Caicedo retained the captain's armband.
The Argentine's progressive passing has been missed in the team - as a fantastic through pass to Cole Palmer in the early stages nearly resulted in a chance for the Blues.
Palmer has just one goal involvement in his last eleven matches, and will have been itching to make an impact. The midfielder netted a famous hat-trick in this fixture two seasons ago, including a last minute winner to give Chelsea a 4-3 victory under Mauricio Pochettino.
Rosenior was forced into an early change, with former United winger Alejandro Garnacho replacing the injured Estevao Willian. The substitute nearly made an instant impact in the box as he got on the end of an angled pass by Pedro Neto, but couldn't make solid contact with the ball.
United held the lower share of possession and had not yet tested the Chelsea goal, but still held somewhat of a threat on the counter, with Matheus Cunha and Bryan Mbuemo ready to pounce.
There was clear defensive organisation off the ball, and they restricted the Blues to just one shot on target in the opening thirty minutes - impressive given their number of defensive absentees.
Bruno Fernandes is on course to break the record for most assists in a single Premier League season, and moved one step closer to the tally with United's opener - getting down the right flank and sending in a cross for Cunha, who bundled the ball past a hopeless Robert Sanchez.
Chelsea switched off defensively; Wesley Fofana was receiving treatment off the pitch, which left the Blues a man down. There was confusion within the six yard box, allowing for Cunha to get his shot off uncontested.
Boos followed the Blues as they left the pitch for half time - the chances were there in the first half, but Liam Rosenior would now need to find ways to motivate his players and encourage a comeback.
There was certainly more urgency by the home side in the second half - Marc Cucurella made a deep run into the box, stretching the United defence, before Moises Caicedo attempted an effort from deep.
This energy revved up the home support, who kept up the noise around Stamford Bridge.
Liam Delap had a quiet first half but came alive following the restart. He struck the bar with a well timed header, and later showed great ball control under pressure - albeit failing to get a shot away.
A rare United break saw Benjamin Sesko drive into the Chelsea half, before laying the ball off to Fernandes. The United captain looked to feed a ball over the top to Cunha, but a nasty deflection required a parry by Sanchez at the near post.
Once again, Chelsea came close - this time through Wesley Fofana, who struck the crossbar from a set piece. Moments later, Cole Palmer won a free-kick in a promising position, and rebounded his effort into the United wall twice.
Garnacho then looked to have won a penalty for his side in the latter stages of the game, with loud cheers going up around Stamford Bridge, but the referee instead awarded a goal kick, deeming the challenge by Noussair Mazraoui a fair one.
Mason Mount was introduced for the visitors, to loud boos, whilst Liam Rosenior opted for defensive switches - Josh Acheampong and Trevoh Chalobah replacing Wesley Fofana and Malo Gusto.
Fresh faces in attack would have been ideal, but Rosenior found himself very limited in options due to injury - Marc Guiu was the only attacking option left remaining at the head coach's disposal.
Chelsea found themselves with a handful of free kicks in the final few minutes of injury time, but both were dealt with effectively by the United defence, and they were able to hold on to a well-fought victory.
The United captain moves to 18 assists for the season - two shy of matching the league record shared by Kevin De Bruyne and Thierry Henry.
Whilst Kobbie Mainoo stuck to Cole Palmer and restricted his fellow compatriot's creativity - Chelsea failed to do the same for Bruno Fernandes, who had space and freedom to find opportunities for the United front line.
He often drifted down the right flank, as he did for Cunha's goal, and looked energetic off the ball to make himself available.