Portsmouth blasted the Premier League title race wide open on Saturday with a 2-1 victory over leaders Manchester United at Fratton Park.
Matthew Taylor's first half strike and a late own goal by Rio Ferdinand ended a run of seven successive league victories for United who managed a late consolation through John O'Shea.
Champions Chelsea, who edged a 1-0 victory over Tottenham Hotspur at Stamford Bridge earlier, are now just three points behind with six games to go.
Portugal defender Ricardo Carvalho scored Chelsea's winner in the second half against Tottenham, leaving Jose Mourinho's side in hot pursuit with 75 points to United's 78.
It is the first time since December that United's lead has slipped below six points and a potential title showdown is beckoning when the sides clash at Stamford Bridge on May 9.
"It's come at a bad time but I expect us to recover," United boss Alex Ferguson told Sky Sports. "Playing with 10 men in Roma on Wednesday told on us today."
Elsewhere, Liverpool strengthened their hold on third place with a 2-1 victory at Reading, while Arsenal fell to their first ever defeat at the Emirates Stadium, going down 1-0 to relegation fighters West Ham United.
United's title hopes in 2003-2004 were dashed by an April defeat at Fratton Park and while this defeat may not prove fatal it will give Chelsea renewed hope of a third straight title.
INSPIRED JAMES
The match proved a tale of two goalkeepers. United's Dutch stopper Edwin van der Saar was culpable over both Portsmouth's goals but David James was inspired at the other end.
Van der Saar could only parry Benjani's fierce shot after 30 minutes and Taylor beat Ferdinand to stab in the rebound.
United launched a wave of attacks but could find no way past James who made superb saves from Cristiano Ronaldo, Paul Scholes and substitute Alan Smith.
There was a comical element to Portsmouth's second goal in the 89th minute. Ferdinand intercepted a Portsmouth move but his pass back to Van Der Saar rolled agonisingly past the on-rushing Dutchman into the net.
O'Shea's reply set up a frantic finale but Pompey, now in eighth, hung on to revive their own hopes of qualifying for the UEFA Cup.
Chelsea's eighth consecutive league victory without conceding a goal was in the balance until the final whistle against a Tottenham side who had been in UEFA Cup quarter-final action against Sevilla just 36 hours before.
Carvalho's strike gave them a barely-deserved lead and they looked jaded after their Champions League exertions against Valencia on Wednesday.
SHARP SAVES
Salomon Kalou missed a sitter while Tottenham came close when Egypt striker Mido, substitute Dimitar Berbatov and French teenager Adel Taarabt all forced sharp saves from Petr Cech.
"Ricardo has been unbelievable this season," Chelsea manager Jose Mourinho told Sky Sports. "I couldn't be happier that he scored. United are lucky they are not playing us next week."
The relegation dogfight took another twist with West Ham's Bobby Zamora stunning Arsenal on the stroke of halftime to earn his side an unlikely 1-0 victory.
West Ham are now two points behind third bottom Sheffield United, beaten 2-1 by Newcastle United, and Charlton Athletic who drew 0-0 with Manchester City on Friday.
Liverpool completed a great week after their 3-0 win at PSV Eindhoven in the Champions League quarter-finals when Dirk Kuyt headed a late winner at Reading to put them five clear of Arsenal.
Fifth-placed Bolton Wanderers are now just two points behind Arsenal after a 3-1 victory at sliding Wigan Athletic, two of the goals coming from Iranian Anderanik Teimourian.
Aston Villa eased towards safety with a 2-1 win at Blackburn Rovers while Mark Viduka scored twice for Middlesbrough in a 4-1 thrashing of bottom club Watford 4-1.