Raheem Sterling was the hat-trick hero as Manchester City opened up a four-point gap at the top of the Premier League, as Pep Guardiola’s team continue their pursuit of an unprecedented quadruple of trophies.

His goals came in the space of 13 minutes at the start of the second-half as City cut loose, having been frustrated throughout the first period on an evening when Watford were threatening to be very awkward opponents.

But there was a huge amount of controversy about the goal which changed the game in the 47th minute when referee Paul Tierney overturned the initial decision of his assistant to flag for offside.

Sterling had the ball in the net when his effort looped over Watford goalkeeper Ben Foster, but he had been in an offside position when Sergio Aguero had touched on Ilkay Gundogan’s lofted pass.

However, the ball had been kicked on to Sterling’s shin by defender Daryl Janmaat, and so the referee was happy to award the goal after a long dialogue with his assistant, much to the dismay of the Watford defenders.

With that, the game changed in City’s favour and the points were never in doubt as the pressure now switches to Liverpool, who face Burnley at Anfield on Sunday lunchtime.

There had been eyebrows raised before the game when the Watford team-sheet revealed seven changes to their starting line-up from the previous game.

Captain Troy Deeney and in-form winger Gerard Deulofeu were both on the bench as manager Javi Gracia appeared to have one eye on next Saturday’s FA Cup quarter-final at home to Crystal Palace.

If Sterling’s first goal was controversial, the second was simple as he tapped in from little more than a yard out as Mahrez squared the ball across.

And the matchball was his on the hour mark when he received David Silva’s pass, cut inside his marker and beat Foster to settle proceedings.

Sterling was then taken off to a standing ovation to keep him fresh for Tuesday's Champions League clash with Schalke as City's quadruple chase continues unabated.

However, it was two of Watford’s subs who combined with their first touches to get the next goal as Deeney nodded down for Deulofeu to score 22 seconds after coming on.