Chelsea lost control of their own destiny in the Premier League title race after crashing to a 2-1 loss to Sunderland at Stamford Bridge on Saturday.
The Blues came into the contest knowing wins in their final four games would be enough to take the title, but the Black Cats frustrated them throughout the match.
Chelsea took an early lead through Samuel Eto'o, but a mistake from keeper Mark Schwarzer presented Connor Wickham with the equaliser six minutes later.
The home side pressed for a winner, but a penalty from Fabio Borini sealed a shock victory for Sunderland—who are fighting to remain in the Premier League.
The Blues travel to title rivals Liverpool next weekend, but not even a victory will be enough if the Reds win their other three games.
Sunderland made a bright start and Wickham, so impressive against Manchester City in midweek, demonstrated great forward play to collect a long ball and lay off for Fabio Borini, who shot wide.
Chelsea's first meaningful chance fell to Willian on 11 minutes. A shimmy bought him a yard of space, but his curling shot lacked power.
Willian was off with his shot, but his distribution brought the opener on 12 minutes. Eto'o shrugged off his marker in the box and volleyed home the Brazilian's corner.
Jose Mourinho has been somewhat grumpy in recent weeks, but Eto'o's dancing celebration on the touchline brought a broad grin to his manager's face.
The Chelsea boss' grin turned to a scowl on 18 minutes as Sunderland drew level. A clever corner routine found Marcos Alonso on the edge of the box. Mark Schwarzer, who was a surprise name on the Chelsea team sheet in place of the ill Petr Cech, fumbled the crisp shot, and Wickham was on hand to bundle home.
Vito Mannone was at fault for Manchester City's late leveller in midweek, and he appeared in a mood to atone as he made decent saves to deny Oscar, Branislav Ivanovic and Mohamed Salah.
When Mannone was beaten shortly before half-time, Sebastian Larsson came to his rescue with a quite brilliant challenge to force Ramires to head wide with the goal gaping.
Chelsea turned defence into attack to near perfection shortly after the restart. After struggling to clear, Ivanovic smuggled the ball to Willian, who surged forward.
He delivered a pass into the path of Eto'o whose first-time shot flew a foot wide of the far post.
Mourinho threw on Demba Ba for Oscar and he passed up a huge chance on 65 when slicing wide following great build-up play from Willian.
Chelsea turned to Fernando Torres to add to their attacking crop, but they were shocked eight minutes from time by a penalty.
Cesar Azpilicueta slipped in possession and in attempting to win the ball back, he felled Jozy Altidore in the box.
Borini was the coolest customer in Stamford Bridge as he rolled his penalty down the middle to beat Schwarzer.
Mourinho had a rueful grin when the ball went in, while his assistant Rui Faria went ballistic, knowing his side's challenge had effectively been ended.
What was also ended was his long unbeaten home record in the Premier League, which had spanned his two spells at the club, and 77 games in total.
Mourinho gave a very brief interview to Sky Sports, in which he said:
"Congratulations to Mike Riley and Mike Dean for making the league what it is," - Mourinho.— Alex Shaw (@AlexShawESPN) April 19, 2014
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