Inter Milan and Juventus face off on Sunday in a clash between Serie A's two title favourites and historic rivals, as Walter Mazzarri begins his second reign at troubled Napoli.

Two points ahead of Juve after 12 matches, leaders Inter have had a dream season so far as they look comfortably Italy's best team and have ensured passage to the Champions League knockouts with two matches to spare.

Now they begin a brutal run of fixtures against a Juve team which has become the distillation of Massimiliano Allegri's hyper-pragmatic style of football -- hard on the eyes and even harder to break down.

With Milan and champions Napoli both faltering Juve are Inter's closest challengers and have the benefit, thanks to last season's accounting controversies, of having no European football clogging up their calendar.

Inter meanwhile take on Juve in Turin before travelling to Portugal to face Benfica and then move on to Naples, admittedly a less intimidating prospect than last season, with Udinese, Real Sociedad and then Lazio all also coming up in the next three weeks.

Simone Inzaghi's team have the league's best attack with 29 goals, led by Lautaro Martinez's incredible tally of 12 in 12, and the best defence with just six conceded.

And with Juve's top scorers -- Dusan Vlahovic and Federico Chiesa -- both stuck on four goals since September their opposition appear to lack the firepower with which to damage an Inter team that Roma boss Jose Mourinho said "should win the league by 20 points".

Inter's CEO Giuseppe Marotta meanwhile has tried to shift the focus onto Juve given their continental commitments, trying to dampen expectations after their last two title challenges fell flat.

"In the medium term Juve are favourites because not taking part in European competition allows you to better plan training sessions and means less physical stress on players," Marotta told reporters on Tuesday.

"We've got a good team, a good manager and solid owners, we've got everything to challenge for trophies. That's what we did last year, let's do the same this season."

Already 10 points off the pace, Napoli have a tricky trip to Atalanta on Saturday as Walter Mazzarri tries to get a derailing title defence back on course.

Napoli have been a club in disarray almost since the season started, with Rudi Garcia quickly proving an unsuitable replacement for Luciano Spalletti, and president Aurelio De Laurentiis turning to a relic of the past has not inspired confidence.

Mazzarri has not worked as a coach in 18 months and has long been a living meme for his excuses for defeats, such as players having diarrhoea, it being Edinson Cavani's birthday, and the rain.

But Napoli were unable to attract a top name and the 62-year-old does at least have happy memories as the man who guided southern Italy's biggest club back to the upper echelons of Serie A over a decade ago.

Napoli are just about in the Champions League positions but an Atalanta win would move them into the top four at the champions' expense.