Arsene Wenger's Invincibles - with that unbeaten 38game league campaign - have been followed, for two seasons, by Jose Mourinho's Incredibles.

Ferguson has always regarded Christmas as the "start of the run-in". Indeed, for five of Manchester United's eight titles gained under him they trailed going into the festive period only to eventually end up in first place. History showed that it's not always best to lead from the front. But Chelsea have been re-writing history. Indeed, since Arsenal were overhauled in 2003 - with United seven points adrift on St Stephen's Day before embarking on a run of 15 wins and three draws - Ferguson's belief has been an idle boast.

The attempts at a traditional charge have only resulted in coughs and splutters as his team hit the vapour trails of those ahead of them.

But that has changed. In the plethora of words uttered by Alan Curbishley on his return to West Ham United last week there was a telling comment on United. Curbishley, during his sabbatical from management, had visited his friend Ferguson at the club's training ground, Carrington, before the season started.

"It was hard to put my finger on it, " Curbishley recalled.

"But I realised then from Fergie and the whole place that they were up for the fight."

West Ham went on to beat United, of course, in Curbishley's first game in charge.

It was a result that was all the more galling for Ferguson as it immediately followed Chelsea's astonishing late comeback against Everton.

From looking at a lead of eight points going into this weekend, United travelled back north a slender two ahead of the champions. It was a blue Sunday indeed and led to the bookmakers shortening the odds on Chelsea.

"This was the most crucial day of the championship race, " claimed Didier Drogba, who scored that winning goal at Goodison Park, and who is the closest Chelsea player to Mourinho and often articulates his philosophies. The striker reasoned that United will have mentally been awarding themselves the greater lead - and that will now take its toll. Drogba could well be right.

If it's a contest of mental strength then it's hard to look beyond Chelsea. But, knowing that he's in a stronger position than for some time, Ferguson will relish all this.

There was certainly a glint as he laughed off Mourinho's jibe that his team can already add another three points to their total - because they will beat United when they visit Stamford Bridge in April.

But whatever happens from now until then - and until the final day fixtures in May - there is something to marvel at Ferguson's appetite and ability to re-fashion his team as genuine contenders.

And also one which is capable of playing wonderful, freeflowing, fluid football that touches the heights far more consistently than Mourinho's awesome creation. Maybe it's also due to Ferguson's belligerent desire to prevent the younger man matching his feat of three successive Premiership titles.

Certain factors have gone United's way - not least the astonishing re-emergence of Paul Scholes and the sustained form of Ryan Giggs.

Also, crucially, Chelsea have changed formation, gearing themselves towards winning the European Cup - with the assumption that they can expect to grind out the Premiership. They have also clearly struggled to integrate the increasingly forlorn Andrei Shevchenko and, to an extent, Michael Ballack, often appearing like a collection of great players dependent on individual brilliance - with Drogba and Michael Essien the greatest of all right now - rather than a team.

But there have also been some bold decisions from Ferguson, testimony to his enduring resilience and know-how, that have worked - not least the shedding of Ruud Van Nistelrooy and the faith placed in Louis Saha who has changed the nature of United's attack, providing greater variety and fluency.

Ferguson's signings last January of Nemanja Vidic and Patrice Evra, on apparently inflated price tags for £7.2m and £5m, have been vindicated with some impressive performances once they adjusted to the Premiership.

Weaknesses do exist. The arrival of Henrik Larsson, a clever bit of business, should address one while Ferguson's continued pursuit of Owen Hargreaves shows the concern he has in central midfield where Michael Carrick, despite his undoubted talent and some good displays, has not yet justified that amazing £18m fee. United have also recalled seven players, including Giuseppe Rossi, although some, such as Jonny Evans, who is wanted by Sunderland, will go back out on loan.

Another foray into the market is expected with a £7m bid for 17-year-old Gareth Bale but he may have to be loaned back to Southampton.

Werder Bremen's Miroslav Klose is trying to engineer a switch to Old Trafford, while Ferguson, who has been promised substantial funds, is also still monitoring developments with West Ham United's unsettled Nigel ReoCoker.

But it must been remembered that as well as United have played, and the impetus that has created, they have simply not pulled clear from the champions who are still not firing consistently.

And, in the second half of last season, Chelsea amassed an unbelievable 49 points. Ferguson's needs to find a way of helping his players to kick on and the most obvious solution is reinforcements. His bench looks far weaker.

Chelsea also have issues.

Mourinho's priority is a defender. That may well be a right-back and Manchester City's Micah Richards, Daniel Alves - although Seville have just extended the latter's deal in response to that recent interest - and Valencia's Miguel have all been considered. But concerns have also been raised about central defence, especially with John Terry's ongoing back problems and Khalid Boulahrouz's unconvincing displays. Further forward and Shaun Wright-Phillips can go - although he still wants to stay - while Salomon Kalou is far from the finished article. A lot depends, as with United, on a handful of key players - especially Drogba - remaining fit.

It makes the punishing Christmas schedule - four games in 10 days - all the more intriguing. Shrewd moves in January, as well as players staying out of the treatment rooms, may prove decisive. But, at last, it appears there is a challenge to the procession of the last three years. And that is something everyone can toast.