Chelsea closed to within two points of Manchester United on Sunday after the champions won 3-2 at Everton while the leaders lost 1-0 at West Ham United in Alan Curbishley's first game in charge of the Londoners.
A stunning 87th-minute goal by Didier Drogba earned Chelsea their win after they twice trailed, while a 75th-minute Nigel Reo-Coker strike handed Manchester United their first away defeat of the season.
A week ago United led the league by eight points but Chelsea's midweek win over Newcastle United in their game in hand and Sunday's results have closed things up as the halfway point looms.
After 18 matches United have 44 points with Chelsea up to 42. Liverpool are third on 31 points after winning 3-0 at Charlton Athletic on Saturday, with Arsenal fourth on 30 after they were held 2-2 at home by Portsmouth.
Tottenham Hotspur climbed to seventh on Sunday when they secured their first away win of the season, 2-1 at Manchester City who were previously unbeaten at home.
Manchester United had not lost at Upton Park since the start of the Premier League in 1992 while Curbishley had never been in charge of a winning team against them in 15 attempts.
Both those runs ended on Sunday as the Hammers finished a turbulent week on a high.
After losing five of their previous six games to slip to third-last in the standings, they sacked manager Alan Pardew on Monday, appointing former Charlton Athletic boss -- and West Ham midfielder -- Curbishley two days later.
As so often happens in such a situation, the change rejuvenated the team.
They had the best chance of the first half when Bobby Zamora broke free in the 32nd minute but shot straight at Edwin Van der Sar, although they needed several good saves by goalkeeper Robert Green to keep a creative United at bay.
The goal was created by a combination of substitute Teddy Sheringham's guile -- a pass threaded through Nemanja Vidic's legs -- and the persistence of Marlon Harewood, who battled past Rio Ferdinand to reach the byline and square the ball.
Reo-Coker, who has been very much at the centre of West Ham's recent troubles with rumours of a transfer request, then burst forward to slide the ball home.
"It has been such a stressful week, not just personally but for the squad," Reo-Coker told Sky Sports.
"It has been a whirlwind, so much has gone on, such a quick change, but we had to make sure we were focused today."
United boss Alex Ferguson bemoaned the timing of his team's visit. "We played really well, we didn't deserve to lose but I think we came here at a time of tremendous upsurge in energy, enthusiasm and optimism," he said.
"If they played like that every week they wouldn't be in that position but that's what happens when a new manager comes in."
Earlier on Sunday, Everton took a deserved lead at Goodison Park in the 38th minute with a Mikel Arteta penalty.
Chelsea equalised four minutes into the second half with a Michael Ballack free kick that crept in off a post and goalkeeper Tim Howard's back.
Everton retook the lead in the 64th minute with a Joseph Yobo header, Chelsea levelled in the 81st with a fiercely struck 25-metre shot by Frank Lampard and, after Drogba hit a post, the Ivorian striker's 35-metre looping shot somehow got through Howard's flailing arms to win it.
"The players had incredible character, they believed until the last moment," said Chelsea manager Jose Mourinho.
