Chelsea's march towards the Premier League title was stalled as goals from Wilfried Zaha and Christian Benteke earned Crystal Palace a shock 2-1 victory at Stamford Bridge on Saturday.
Defeat ended Chelsea's run of 13 successive home wins and enabled second-place Tottenham Hotspur to cut their lead to seven points with nine games remaining.
Antonio Conte's side had taken the lead through Cesc Fabregas's fifth-minute effort, but the visitors hit back first through the outstanding Zaha, four minutes later, and then Benteke.
The visitors then held on to claim a fourth successive win that strengthened their bid to avoid relegation.
Sam Allardyce had warned his side would have to score first if they were to have any chance of getting something out of this game.
On that basis, the Palace manager must have feared the worst when Fabregas put the home side ahead.
Eden Hazard had been doubtful for this game, having sat out the trip to Stoke City and both of Belgium's matches last week with a calf problem.
But the winger quickly confirmed he was back to his best by tormenting Palace right-back Joel Ward before drilling in a low cross that was turned home at the near post by Fabregas.
The signs were ominous for Allardyce's men, whose improved recent form has owed more to resilient defensive work rather than their attacking prowess.
Had Chelsea been allowed to settle and establish their superiority after taking the lead, Palace's confidence might have started to fade.
Instead, Allardyce's side struck back quickly through Zaha, who has been a constant threat during Palace's recent upturn.
Like Hazard, the Palace winger found himself the subject of debate last week, in Zaha's case over his decision to commit his international future to Ivory Coast and not England.
- Nerveless Benteke -
England manager Gareth Southgate expressed disappointment about his defection to the land of his birth, prompting angry responses from Zaha's agent and Steve Parish, the Palace chairman.
Zaha, though, showed his focus was entirely on club matters when he revived Palace's hopes with a superb individual effort.
The former Manchester United winger drifted past two Chelsea defenders before placing a shot across Thibaut Courtois after being released by Benteke's flick from Ward's cross.
Two minutes later, Palace took control of the game after the Chelsea defence was again found wanting.
This time Benteke led a breakaway after winning the ball in the Palace half, advancing deep into Chelsea territory before benefiting from a fortunate ricochet after being challenged by David Luiz.
The ball rolled into the path of Zaha, who had time and space on the left of the Chelsea box to play a return pass to Benteke, who produced a nerveless finish, lifting the ball over the prone Courtois.
The onus was now on Chelsea to force their way back into the game and Conte's side inevitably dominated possession for the remainder of the first half.
They were unable, however, to find a way through the combination of Palace's determined defenders and goalkeeper Wayne Hennessey.
The Wales international produced a succession of fine saves, including one with his legs to repel Costa's volley.
The pattern of play was maintained in the second half, with Chelsea enjoying an even greater share of possession.
But Palace still had chances to extend their lead, with Zaha denied by a good Courtois save before two Yohan Cabaye free-kicks almost led to a third goal.
By the time Costa directed a close-range header wide from Hazard's cross, Chelsea's hopes of rescuing a draw had disappeared.