Middlesbrough applied the brakes to Chelsea's Premier League title express in spectacular fashion on Saturday by humbling the champions 3-0 at the Riverside.
It was Chelsea's second league defeat of the season and only the third since coach Jose Mourinho took charge in 2004. It also equalled the worst defeat of Mourinho's coaching career.
Manchester United took advantage of Chelsea's rare slip by cutting the London club's lead to 12 points with a 3-1 victory at second-from-bottom Portsmouth. Cristiano Ronaldo scored twice before halftime.
Third-placed Liverpool won 1-0 at Wigan Athletic to end a run of three defeats in four games, while fifth-placed Arsenal needed a stoppage time equaliser from Brazilian Gilberto to salvage a 1-1 home draw with Bolton Wanderers.
Relegation-threatened Boro, without a home league win for six matches, made a perfect start when Fabio Rochemback punished some slack defending to score inside two minutes.
Chelsea were hampered by the loss of William Gallas to injury and they slipped further behind on the stroke of halftime when Stewart Downing's shot beat Petr Cech.
Any hopes of a Chelsea comeback vanished after 67 minutes when Nigerian Aiyegbeni Yakubu powered past defender John Terry and rifled a shot past Cech.
It was Chelsea's worst defeat under Mourinho and came against a team hammered 4-0 at home by Aston Villa last week and 7-0 by Arsenal last month.
"We lost at Manchester City and we didn't deserve it, we lost at Manchester United and didn't deserve it, we lost today and did deserve it," Mourinho told the BBC, referring to the only other league defeats during his Stamford Bridge reign.
Chelsea have 66 points from 26 games with United on 54 and Liverpool on 48 with a game in hand. Tottenham Hotspur, who play bottom club Sunderland on Sunday, are fourth with 44 points.
WORRYING SLUMP
European champions Liverpool handed recently returned striker Robbie Fowler his first start for the club since 2001 alongside Fernando Morientes.
But they were indebted to Hyypia to end a worrying slump that had brought just one point and one goal in their last four games as their marathon season began to take its toll.
The Finland defender volleyed in after 29 minutes of his side's first league visit to Wigan.
"We worked very hard and showed a lot of character after the midweek defeat by Charlton," Liverpool manager Rafa Benitez told Sky Sports. "It was an important win."
United ran riot in the first half. Ruud van Nistelrooy nodded in his 22nd goal of the season after Ryan Giggs had hit the crossbar following a dazzling run. Ronaldo then hit a vicious shot past Dean Kiely and struck again to make it 3-0.
Portsmouth dominated the second half and pulled one back through Matthew Taylor but are now eight points from safety.
"If Chelsea have a bad spell, who knows what might happen, United boss Alex Ferguson told Sky Sports.
Injury-hit Arsenal looked to be heading for a third defeat of the season against Bolton after Kevin Nolan lobbed the visitors ahead at Highbury in the 11th minute.
Arsenal had forward Jose Reyes carried off on a stretcher before halfime as a result of a poor tackle by Abdoulaye Faye, but they grabbed a deserved equaliser when Gilberto converted a cross by Cesc Fabregas.
Everton's revival continued with a 1-0 home victory over Blackburn Rovers despite playing for 80 minutes with 10 men and having three goals disallowed.
Goalkeeper Iain Turner, making his league debut, was sent off for deliberate handball but James Beattie headed the only goal in the first half.
Fulham thrashed West Bromwich Albion 6-1, Heidar Helguson and Collins John scoring two apiece, while managerless Newcastle United beat Aston Villa 2-1.
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