Manchester: Cristiano Ronaldo inspired Manchester United's four-goal rout of bottom-club Watford to maintain Sir Alex Ferguson's side's six-point advantage over Chelsea at the head of the Premiership table.

Refreshed after a week's holiday in Dubai, the Portuguese winger chose the final day of the transfer window to demonstrate why United have reacted so angrily to reported interest in him from Real Madrid.

From the moment he set United on their way with the 20th minute opener, Ronaldo proved to be simply unstoppable and rounded off a superb individual display by setting up Wayne Rooney for the fourth goal after a Lloyd Doyley own-goal and Henrik Larsson strike had given Ferguson's side an impregnable lead.

With Chelsea beating Blackburn Rovers, it was vital United won to maintain their six-point gap over the London club after squandering a chance to extend that advantage to nine points when they lost at Arsenal in their last league outing.

United skipper Gary Neville said last week that Ferguson's side were already looking ahead to the weekend visit to Tottenham Hotspur as the next decisive fixture in their bid to claim the Premiership crown for the first time in four years.

That view looked shortsighted as United initially struggled to click into gear but once Ronaldo had fired the home side into a 20th minute lead with a typically swaggering penalty, there was never any doubt this would be a routine victory for the league leaders.

Watford had only themselves to blame for the opening goal with centre-back Jay DeMerit conceding the spot-kick by impeding Ole Gunnar Solskjaer as the Norwegian attempted to reach Wayne Rooney's knock-down on the edge of the six-yard box.

Lifted by his 14th goal of the season, the Portugese began to demonstrate his full repertoire of tricks but United struggled to build on the lead with centre back Nemanja Vidic twice sending powerful free headers over the bar.

Against a better side those misses might have proved wasteful but Watford offered precious little as an attacking force although United keeper Tomasz Kuszczak, deputising once again for the rested Edwin van der Sar, was fortunate to see Chris Powell's sliced cross deflect to safety off the far post in the 36th minute.

That, though, was to be the visitors last real opportunity and after the break Ferguson's side turned the game into little more than a training work-out.

They were helped on their way once again when DeMerit's central defensive partner Doyley headed Kieran Richardson's left wing cross past his own goalkeeper for United's second on the hour.

But United needed no further assistance as they completed the victory with two goals that epitomised the confidence that is running through Ferguson's side at the moment.

First Larsson carved open the visiting defence with an exhilarating exchange of passes that resulted in the Swede being sent clear on Watford keeper Richard Lee before side-footing home to claim his second goal for the club just five minutes after appearing as a 65th-minute substitute.

Then sixty seconds later Rooney raced onto Ronaldo's elegant chip to loop the ball over the advancing Lee and take his personal tally to four goals in three games with a strike reminiscent of his superb match-winner in the weekend FA Cup victory over Portsmouth.

Ronaldo almost added to the rout when he struck the post late on but by then Ferguson's side had already got their title bid well and truly back on course.