Sergio Aguero underlined his value to Manchester City with a hat-trick in a dramatic second half comeback that maintained his side's interest in chasing an unprecedented quadruple of domestic and European silverware.
After a first-half performance which left manager Manuel Pellegrini confessing that he would have liked to have substituted all 11 players at the interval had the option been available, Aguero proved the difference between a City side that had £84m of signings on the bench and a Watford 18 that cost a grand total of £500,000 – all on Troy Deeney.
Deeney himself added to an early goal from Fernando Forestieri to bring Watford a deserved 2-0 half-time lead before Pellegrini took off the woeful Micah Richards and Jack Rodwell, adding Pablo Zabaleta and Vincent Kompany to his defence and pushing Martin Demichelis into midfield.
But it is in Aguero, scorer of 25 goals in 24 games and six in the four matches since his return from a one-month injury lay-off, Pellegrini possesses the player who renders talk of that quadruple plausible.
"Sergio's a top player," his relieved manager said. "The first idea was for him not to play 90 minutes because he has been playing 30, 45, 60 and the idea was for him to play 60 or 70. But he had to play the whole match. He made the difference."
Although none of the Aguero goals would feature in his most outstanding efforts, the trio highlighted the fact that, once in the opposition penalty area, there may not be a more prolific converter of chances in English football.
On the hour, the Argentine was on hand to steer the ball into an open net after Jonathan Bond had blocked well at close range from Edin Dzeko.
Eighteen minutes later, Aguero finished superbly, steering the ball over Bond into the top corner after Dzeko had picked him out.
He completed his team's and his own scoring deep into injury time, rising at the far post to head home a cross from Jesus Navas that left the scoreline flattering the home side.
Prior to that, on 87 minutes, the tie had been effectively settled in an unfortunate manner when Kolarov's speculative shot from over 25 yards squirmed through the grasp of Bond and over the line. It was an unkind manner for the Watford goalkeeper and his team to exit the competition but a sign, perhaps, that on top of City's financial advantage, they can also count on a large slice of luck when needed.
"It's the same question every week," said Pellegrini when quizzed about the prospect of four trophies coming his way in his first season at the Etihad. "I don't know what will happen in the future. I'm absolutely sure if we play as we did in the first half, it is impossible."
The beaten Watford manager Giuseppe Sannino, however, could be less cautious in analysing the team that had just knocked him out of the competition. "I used to watch Manchester City on TV, tonight I saw them from the touchline," the Italian said. "They have a lot of very, very good players. Yes, I think they can compete for all four titles they are looking for. But you know football is very strange so they have to be focused every single game."
City were certainly not focused during a first half in which a three, or even, four-goal deficit would not have been unreasonable, and this for a team who have trailed at home to English opposition for a grand total of eight minutes this season – one to Everton, seven minutes to Liverpool.
After 20 minutes, Marco Faraoni slipped the ball forward for Deeney whose through pass was perfect for Forestieri to run on, compose himself, and give Watford the lead with a shot through the legs of Costel Pantilimon.
Once more exposing flaws in the home defence after 28 minutes, Forestieri and Ikechi Anya made swift, slick passes to play in Deeney who, again, showed great composure in comfortably beating Pantilimon.
There was also a reasonable penalty appeal after Anya went down under a challenge from Pantilimon and Daniel Pudil ended the first half by lashing a shot into the side netting.