Fulham scored twice in the final 10 minutes to end Chelsea's 12-game unbeaten run in all competitions with a stunning 2-1 Premier League win at Stamford Bridge.
Substitutes Harry Wilson and Rodrigo Muniz shocked the Blues to secure Fulham's first away win at their west London rivals since 1979.
Defeat leaves second-placed Chelsea still four points adrift of leaders Liverpool, who have two games in hand over their nearest challengers, starting with the visit of Leicester later on Thursday.
Victory lifts Fulham up to eighth, level on points with champions Manchester City, and within just one point of fifth-placed Newcastle to further their European ambitions for next season.
"It is a privilege to give our fans this happiness," said Fulham boss Marco Silva.
"Second half the reaction was top. It is not a coincidence that we are the team in the Premier League that has got the most goals from players off the bench."
Chelsea boss Enzo Maresca has insisted in recent weeks that his young side are not yet cut out for a title challenge despite their fine form.
And the home side proved their manager's point after failing to build on a dream start given to them by the class of Cole Palmer.
There did not look to be much on when Levi Colwill played the ball into Palmer's feet about 40 yards out.
But the England star burst into life, turning away from Andreas Pereira and gliding past Sasa Lukic towards the edge of the box.
Palmer then applied the finish, nonchalantly passing the ball through the legs of defender Issa Diop and into the corner of Bernd Leno's goal.
It was Palmer's 26th league strike of 2024, beating the Chelsea record for Premier League goals scored in a calendar year set by Jimmy Floyd Hasselbaink in 2001.
"We were winning for most of the game, but to lose at the end is difficult," said Palmer.
"Second half we lost a bit of control. They're a good transition team and like to run direct, we got caught up in their game and paid the price."
Palmer almost bagged a second moments after opening the scoring, with a nutmeg on the edge of the box followed by a low drive which was well kept out by Leno.
Fulham's German goalkeeper also made important saves to prevent Marc Cucurella and Enzo Fernandez adding to Chelsea's lead either side of halftime.
Silva's men have now taken points from Liverpool, Arsenal and Chelsea this month and the Portuguese's changes turned the tide in the visitors' favour.
Eight minutes from time another substitute Tom Cairney's cross was headed across goal by Timothy Castagne and Wilson glanced in at the far post.
Both sides had chances to win it in a wild finale, but it was Fulham who took theirs in the 95th minute to claim a famous victory.
Lukic surged forward from midfield and squared for Muniz to coolly slot into the bottom corner in front of the delighted travelling support.