David Nugent scored his first goal since January to give Leicester City a slender advantage going into Sunday's play-off semi-final second leg at Vicarage Road.
The City striker headed home Anthony Knockaert's free-kick to nudge City ahead in their bid to reach Wembley and the Premier League.
City, who have been at Wembley four times before in the Championship play-offs, must fancy their chances of another visit after this victory.
Watford beat City twice during the season, the second time just two weeks ago at the King Power Stadium, but this was a very different looking Leicester in the first half.
long spells
Buoyed by the way they reached the play-offs, with that excellent win at Nottingham Forest, City looked like a side that realised they had been handed an unlikely opportunity, something that had seemed impossible after that defeat to the Hornets.
Cheered on by a fantastic home support of nearly 30,000, City started superbly and pegged the visitors back in their own half for long spells in the first half and created a host of chances.
But were unable to take them, something they have paid the price for many times during games throughout this season.
Lloyd Dyer and Anthony Knockaert were the architects in the first half, but City were wasteful with David Nugent and Michael Keane failing to hit the target when well placed.
But it wasn't just City, thankfully. The game became very scrappy in the second period, with both sides making mistakes and squandering possession.
The Hornets should have stung City on the counter-attack but Matej Vydra scuffed his shot when clean through on goal.
It was then Alex Geijo who headed over form close range at the far post.
After looking so confident and powerful in the first period, City looked nervy, and the visitors had grown into the tie, and looked the most likely to score.
But then Knockaert swung in a delightful free kick and Nugent, the man who had not scored in 16 games, flicked his header superbly into the far corner to give City that slender advantage.
However, City will know that the job is only half done, but Nugent's strike has at least nudged them a little closer to Wembley.
