Chelsea manager Antonio Conte declared his side's Premier League title defence over after Marko Arnautovic's goal condemned the champions to a 1-0 loss at West Ham United on Saturday.
Chelsea's first defeat in eight league games left them 11 points adrift of leaders Manchester City, who visit second-place Manchester United in Sunday's derby.
"We have to consider that this is the fourth game we've lost in 16 games," Conte told Sky Sports at the London Stadium.
"In my previous experience, it won't be possible to fight for the title."
Elsewhere, Tottenham Hotspur ended a run of four games without victory with a thumping 5-1 home win over Stoke City at Wembley.
Having previously been beaten by Burnley, Manchester City and Crystal Palace, Chelsea have lost just one fewer game than in the entirety of their triumphant 2016-17 campaign.
Austria international Arnautovic struck in the sixth minute, neatly exchanging passes with Manuel Lanzini and cutting inside Cesar Azpilicueta before curling a low shot past Thibaut Courtois.
It was West Ham's first win in nine league games and confirmed the progress they have made under new manager David Moyes, who saw his side narrowly beaten 2-1 at Manchester City last weekend.
"The win, the spirit of the team, how they stuck together and more importantly a lot of the basics were done well today," said Moyes. "All that really pleased me."
West Ham remain in the relegation zone, but they are now level on points with fourth-bottom West Bromwich Albion, who lost 1-0 at Swansea City in Alan Pardew's second game as manager.
Wilfried Bony scored for the second game running to lift Swansea off the foot of the table.
Spurs claimed a fourth successive four-goal win over Stoke, making Mauricio Pochettino's side the first team to have won by four goals in four consecutive English top-flight meetings with the same opponents.
Son Heung-Min's cross was deflected in via Stoke defenders Kurt Zouma and Ryan Shawcross to put Spurs ahead and Son made it 2-0 by rattling in a pass from Dele Alli in the 53rd minute.
Harry Kane scored twice - a header from a Ben Davies cross and a left-foot curler - to take his tally for the season to 18 in all competitions and Christian Eriksen completed the rout.
Spurs moved above Arsenal into fifth place on goal difference ahead of their local rivals' trip to Southampton on Sunday.
Stoke, who replied through Shawcross, have now lost 1,000 top-flight games.
Burnley are level on points with the two north London heavyweights after Scott Arfield scored in a 1-0 home win over Watford, who had Marvin Zeegelaar sent off for a studs-up foul on Steven Defour.
In-form Leicester City climbed to eighth after a luckless 86th-minute own goal by Ayoze Perez - who prodded the ball into his own net as he attempted a tackle - earned them a 3-2 win at Newcastle United.
Riyad Mahrez cancelled out Joselu's fourth-minute opener for Newcastle and Demarai Gray made it 2-1 to Leicester, but after Dwight Gayle levelled, Perez's late blunder gifted the visitors the points.
Crystal Palace manager Roy Hodgson rebuked Christian Benteke after he squandered a stoppage-time penalty in a 2-2 draw at home to Bournemouth that left his side bottom.
Jermain Defoe put Bournemouth in front and then equalised with a brilliant dipping volley after Luka Milivojevic's penalty and a Scott Dann effort had put Palace in charge.
Palace were awarded a last-gasp penalty when Charlie Daniels felled Wilfried Zaha, but after snatching the ball from Milivojevic, Benteke saw a tame effort repelled by Asmir Begovic.
"It was a unilateral decision," Hodgson said. "No-one on our team was able to wrest the ball from him."
Steve Mounie's first goals since the opening day earned Huddersfield Town a 2-0 win over fellow promoted team Brighton and Hove Albion as David Wagner's side snapped a four-game losing streak.
(Arfield 45) Watford 0
Crystal Palace 2
(Milivojevic 41-pen, Dann 44) Bournemouth 2
(Defoe 10, 45+3)
(Mounie 12, 43) Brighton 0
(Bony 81) West Brom 0
(Shawcross 21-og, Son 53, Kane 54, 65, Eriksen 54) Stoke 1
West Ham 1
(Arnautovic 6) Chelsea 0
(Joselu 4, Gayle 73) Leicester 3
(Mahrez 20, Gray 60, Perez 86-og)