A superb injury-time goal by substitute Robin van Persie saved Arsenal from a second successive defeat on Saturday as the young Dutchman earned the champions a 2-2 home draw with Southampton.
Arsenal led through Thierry Henry but two headers by Rory Delap in the last 10 minutes gave Southampton, who had not scored in their previous four league games, a shock lead.
Van Persie's late effort left Arsenal top only on goal difference from Chelsea, who won 4-1 at West Bromwich Albion with goals by William Gallas, Eidur Gudjohnsen, Damien Duff and Frank Lampard.
Manchester United, who revived their title hopes with last week's 2-0 win over Arsenal, came down to earth with a bump as they lost by the same score at Portsmouth after goals by David Unsworth, from the penalty spot, and Aiyegbeni Yakubu.
Arsenal remain top with 26 points, the same as Chelsea, while third-placed Everton lost ground on 23 after a 1-1 home draw with Aston Villa.
Middlesbrough moved up to fourth on 18 with a 2-1 win at Charlton Athletic. Bolton Wanderers, also on 18, host Newcastle United on Monday while Manchester United have 17.
Liverpool are level with United after drawing 2-2 at Blackburn Rovers, bottom at start of play.
Czech striker Milan Baros scored Liverpool's second-half equaliser after the visitors' French striker Djibril Cisse was carried off with a suspected broken leg.
Arsenal were odds-on favourites to embark on a new winning run against a Southampton team who had managed just one goal and one point in their five away games this season.
MISSED PENALTY
The champions, on the wrong end of a penalty decision last week when their 49-game unbeaten league run was ended by United, were awarded one after half an hour when Dennis Bergkamp was tripped but Henry sent the spot kick against a post.
The Frenchman was then denied by goalkeeper Antti Niemi and clipped a post before finally finding the net in the 67th minute after a neat through ball by Bergkamp.
Southampton levelled with a firm header by an unmarked Delap from a Neil McCann corner in the 80th minute and Highbury was stunned when the same pair combined to put them ahead five minutes later.
But Van Persie, the 21-year-old who arrived from Feyenoord in May, turned in a crowded box to fire home the equaliser seven minutes after coming on.
"When you don't know defeat for a long time, of course it becomes a big shock and it was not out of our system today," said Arsenal manager Arsene Wenger.
SEVERE PRESSURE
Chelsea weathered some severe pressure at The Hawthorns but grabbed the lead in first-half injury time through Gallas.
It was a different story after the break as the Londoners, inspired by substitute Arjen Robben, cut loose.
They got their second in the 51st minute when Gudjohnsen headed his fifth goal in four games and though West Brom hit back through Zoltan Gera, Duff and Lampard completed the victory.
Chelsea boss Jose Mourinho was not sure what to think after his side's Jekyll-and-Hyde display.
"The first half was too bad to be true," he said. "But the second half was beautiful.
"With the other results in the top four it was a fantastic day."
Manchester United missed a hatful of chances at Fratton Park and paid a high price by losing there for the second successive season.
Rio Ferdinand pulled down Yakubu in the box and Unsworth lashed in the resulting penalty in the 53rd minute.
United roared back and laid siege to the home goal. Ronaldo hit a post with a header, while Wayne Rooney and Paul Scholes also went close.
But Portsmouth broke in the 72nd minute and a deflected header by Yakubu secured the win.
"It's a kick in the teeth after last week's performance," said United boss Alex Ferguson. "Last weekend's result is absolutely destroyed."
In the day's other games Fulham beat Tottenham Hotspur 2-0 and Crystal Palace won 1-0 at Birmingham.
