Sunderland condemned Chelsea to their first home Premier League defeat under Jose Mourinho with a stunning 2-1 victory on Saturday that took Liverpool a step closer to the title.
Chelsea were bidding to take over from Liverpool at the summit, but their loss to the league's bottom club means that Brendan Rodgers's side can now open up a five-point lead if they win at Norwich City on Sunday.
Samuel Eto'o put Chelsea ahead after only 12 minutes, but Sunderland equalised quickly through Connor Wickham and claimed victory thanks to an 82nd-minute penalty from on-loan Liverpool striker Fabio Borini.
Having held third-placed Manchester City to a 2-2 draw in midweek, Sunderland have emerged as this season's kingmakers, and victory at Stamford Bridge gave their chances of avoiding relegation a sizeable boost.
Gus Poyet's side, who had previously gone nine games without a win, finished the day three points from safety and also have a game in hand to play.
Chelsea took the lead in the 12th minute, Eto'o volleying in from Willian's left-wing corner, but it took Sunderland only six minutes to draw level.
Mark Schwarzer, deputising in goal for the unwell Petr Cech, spilled a long-range shot from Marcos Alonso and Wickham pounced to clip home the rebound.
Chelsea's response was not slow in coming and visiting goalkeeper Vito Mannone had to produce smart saves to thwart Branislav Ivanovic, Nemanja Matic, Mohamed Salah and Willian before half-time.
Chelsea's frustrations told when Ramires lashed out at Sebastian Larsson with his arm, catching the Sunderland midfielder in the face, but referee Mike Dean appeared not to see the incident.
The Chelsea onslaught continued in the second half, with Eto'o curling just wide and substitute Demba Ba miscuing from Willian's cut-back, but the goal would not come and there was worse to follow.
In the 81st minute, Cesar Azpilicueta was penalised for sliding in on Jozy Altidore and Borini, a former Chelsea striker, calmly beat Schwarzer from the spot.
It may prove to be the goal that delivers the title to Anfield and Chelsea's anger was clear, with assistant coach Rui Faria needing to be restrained as he berated Dean.
- Spurs halt Fulham charge -
Earlier, Fulham squandered an opportunity to escape the Premier League relegation zone after going down 3-1 at London rivals Tottenham Hotspur.
Felix Magath's improving side had won their two previous games and they reacted impressively after falling behind to a close-range Paulinho strike in the 35th minute, with Steve Sidwell equalising almost immediately.
However, second-half goals from Harry Kane and Younes Kaboul secured victory for Tim Sherwood's Spurs, with Sidwell seeing a late penalty saved by Spurs goalkeeper Hugo Lloris.
"If we had got any points here, it would have been an extra point for us," said Fulham manager Magath, whose side ended the day two points from safety.
"We have to win our home games and I think we can manage it."
Fulham slipped one place to 19th after Cardiff City moved above them on goal difference by drawing 1-1 at home to Stoke City.
Marko Arnautovic gave Stoke the lead from the penalty spot in first-half injury time, but Peter Whittingham earned Cardiff a draw with a penalty of his own after Steven Nzonzi tripped Fraizer Campbell.
Cardiff manager Ole Gunnar Solskjaer was unhappy about Stoke's penalty, which was awarded for a foul on Peter Odemwingie by Kim Bo-kyung.
"Stoke's was not a penalty," said the Norwegian. "All the players were incensed, but it was important we kept cool and tried to do the right things."
Elsewhere, an ice-cool stoppage-time penalty by Wilfried Bony gave Swansea City a 2-1 victory at Newcastle United that took the Welsh club six points clear of the relegation zone.
Aston Villa spurned a chance to put clear daylight between themselves and the bottom three in a 0-0 draw at home to Southampton, while a 59th-minute Mile Jedinak penalty saw Crystal Palace win 1-0 at West Ham United.