Aguero scored twice to draw City level before Aleksandar Kolarov claimed the decisive 87th-minute goal with a speculative shot that goalkeeper Jonathan Bond allowed to squirm through his body and over the line.

Aguero added gloss to a flattering scoreline, and completed his treble, deep in injury time when he rose to head in a cross from Jesus Navas.

But the Championship visitors had appeared on the brink of an upset thanks to first-half goals from Fernando Forestieri and Troy Deeney -- the only player in their 18-man squad for whom they paid a transfer fee.

From kick-off, the mid-table second-division side played with a degree of organisation, discipline and confidence that few Premier League teams have displayed at the Etihad Stadium this season.

Forestieri placed an early free-kick into the defensive wall, Ikechi Anya had a penalty appeal waved away, and Forestieri wasted a glorious Watford counter-attack by miscueing a pass to Marco Faraoni.

And after 20 minutes, City fell behind for just the third time against English opposition this season.

Faraoni played a clever ball from the right-back position and Deeney's perfect flick freed Forestieri, who produced an accomplished finish through the legs of City goalkeeper Costel Pantilimon.

City had previously trailed to English clubs for a grand total of eight minutes at the Etihad this season -- Everton having held the lead for one minute and Liverpool for seven on City's home turf.

But there was no instant remedy on this occasion and, after 27 minutes, Forestieri almost doubled the lead when he created space for a low shot that flew just wide.

The miss did not matter, for one minute later Watford made it 2-0 with a goal of which City and their galaxy of attacking stars would have been proud.

With City right-back Micah Richards nowhere to be seen, Watford attacked down his flank, with Forestieri and Anya exchanging swift passes to put in Deeney, whose ability to finish past Pantilimon never looked in doubt.

Watford might even have claimed an implausible third goal just before the interval when Faraoni's cross from the right found Daniel Pudil, who shot powerfully into the side-netting.

City manager Manuel Pellegrini sent on Vincent Kompany for the anonymous Jack Rodwell at the interval and replaced the hapless Richards with Pablo Zabaleta, pushing Martin Demichelis into midfield.

It was a move that brought about a gradual improvement. Demichelis almost connected with a Kolarov corner and Edin Dzeko played a one-two with Navas, only to then get the ball stuck in his feet.

On the hour, however, City were back in the tie as Aguero added to his sizeable tally for the season.

Kolarov's left-wing cross was met by Dzeko, who drew a solid block from Bond, only for the ball to break to the unmarked Aguero, who swept home from eight yards.

There was ample possession for City and in the 79th minute Aguero claimed his second goal, which was carved out by a blistering run on the counter-attack by Navas.

The Spaniard's pass was turned on to Aguero by Dzeko and the Argentine equalised with a superb curling shot into the top corner.

Kolarov deceived Bond to put City in front before Aguero added the coup de grace with his 25th goal of the season.