The Premier League’s title race has its unanticipated twist. Chelsea, untouchable for so long at the top and a side that had won 13 games in a row at home, succumbed here to a team who have spent almost the entire campaign choked in the grip of a relegation battle. For the second season in succession, this corner of the capital has been conquered by Crystal Palace. From nowhere, the first seeds of doubt may have crept into the leaders’ procession.

This was a far more eye-catching victory than last season’s success given its context, a team from the lower reaches having checked the Premier League’s previously irrepressible force. Even Palace’s staff and players appeared genuinely startled in the aftermath, Wilfried Zaha catching his breath after another performance to leave the Football Association privately cursing in regret to announce he was stunned.

Sam Allardyce marched into his post-match media conference to joke: “That shocked you all, didn’t it?” He added that with his players four points clear of the cut-off this was no time for celebration, though the din from the away dressing-room, the dance music interspersed with choruses of “Mamadou Sakho, Sakho, Sakho”, suggested otherwise.

Chelsea’s players, more sombre in a neighbouring room, will still be perplexed as to how they had run aground on the Liverpool loanee. Or had been denied so consistently and thrillingly by Wayne Hennessey, a player who has been much maligned throughout his Palace career but is suddenly in the form of his life.

The hosts’ had whipped up frantic pressure almost from the moment they fell behind in the 11th minute, but as the visitors refused to wilt desperation had rather set in. Too many centres were intercepted by Palace players, too many shots belted close enough for Hennessey to claim. Composure drained. Antonio Conte was left hopping in anguished frustration when Diego Costa met Eden Hazard’s cross on the edge of the six-yard cross, but with the goalkeeper resigned he steered a header wide.

The Italian has not been used to picking over losses since arriving in England last summer and looked utterly exhausted as he conducted his own assessment. He has been warning for months that the title is not yet Chelsea’s, with this perhaps a timely reminder to his players of the peculiar quirks of this race.

Tottenham Hotspur have trimmed the gap to seven points – still a relative chasm – but Manchester City are visitors on Wednesday. The recovery must be swift, the sense of alarm that infected much of their play exorcised immediately. Once they had taken time to review this occasion, Conte and his staff will take solace from their attacking vim and vigour, even if the opportunities had been fluffed.

They converted one of them, Cesc Fàbregas flicking in off the near-post early on from Hazard’s delivery, and must briefly have anticipated another stroll against a team near the foot. Palace had not conceded since mid-February but, as Allardyce had made clear, their game-plan was not built for a chase. That merely made the manner of their riposte all the more remarkable. Zaha, inevitably, sparked their rally, squeezing space from a dithering César Azpilicueta and Marcos Alonso just outside the penalty area before fizzing a low shot back across a startled Thibaut Courtois and into the far corner. The speed and accuracy of the shot appeared to take the goalkeeper by surprise. Zaha had no right to gain such power from a cramped position between opponents. It was as if Chelsea had not sensed the danger.

They were still dawdling a little over a minute later when Christian Benteke, a striker so shorn of confidence over recent weeks, bustled forward from the halfway line. David Luiz attempted to stifle his progress only for the ball to deflect to Zaha, sprinting up in support. The Ivorian’s return pass found Benteke alone in front of Courtois, with the striker calmly waiting for his compatriot to go to ground before lifting a delightful finish into the gaping net. It was his first league goal since the end of January.

“Instead of their heads going down, which is what I saw when I first arrived, I saw a massive response,” said Allardyce. “Determination and will to get back into the game ... the two goals we scored put Chelsea in shock. It was an outstanding result.”

For all that, so much thereafter hinged upon Hennessey’s brilliance and Sakho’s rugged refusal to grant Chelsea a sight of goal, Zaha’s pace and trickery also offered Palace hope on the break.

“There’s a man on the other side, in Hazard, who is an outstanding individual like Wilf and his ability is something Wilf could look at to try and match,” said Allardyce.

He has grown infuriated discussing the winger’s future at the club with constant talk of Tottenham’s long-standing interest in the 24-year-old. “Wilf is already realising his potential with the way he’s playing. Potential is becoming a reality.”

Palace’s only disappointment was a potentially serious knee injury suffered by Scott Dann, a half-time replacement for the injured James Tomkins, but their reshaped back-line repelled everything flung at them over the frenetic final exchanges.

This was a result to fire hope that relegation can be staved off. It was also a Chelsea defeat to preserve a degree of intrigue in the title chase.

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Facebook Twitter Pinterest Palace fans celebrate after Benteke’s goal. Photograph: John Patrick Fletcher/Action Plus via Getty Images