Leicester 1-0 Watford
David Nugent's first goal in 17 games gave Leicester a slender and deserved lead over Watford after an open Championship semi-final first leg.
The Foxes dominated a gripping first half with a pacy and powerful display but missed a couple of fine chances.
Watford improved after the break and Matej Vydra and Alex Geijo both wasted glorious opportunities.
But Nugent headed in Anthony Knockaert's late free-kick to secure the win ahead of Sunday's second leg.
It was the striker's 15th goal of the season and, although it is his first since January, it could prove to be well worth the wait for a side whose play-off dream seemed to be over less than a week ago.
The Foxes were second in the table in February but a dismal end-of-season run saw them win just once in their final 12 games before last Saturday.
However, having played with such freedom and imagination in the thrilling play-off clinching victory over East Midlands rivals Nottingham Forest, Leicester started brimming with confidence.
And once again the tremendous pace, direct running and trickery of Lloyd Dyer and Saturday's match-winner Knockaert was the standout feature of their attacking play.
Watford, the Football League's second-highest top scorers this season with 85 goals, were rattled, uncharacteristically sloppy, and barely got out of their half in the opening 15 minutes.
Nugent tested fit-again Hornets goalkeeper Manuel Almunia with a cheeky back-heel and had a decent strike blocked, and Ritchie De Laet headed straight at the former Arsenal man from point-blank range.
Watford's incredible season was in danger of being forgotten as they struggled to cope with the onslaught and, seemingly, the crushing disappointment of Saturday's failure to secure automatic promotion.
But having weathered the storm in blustery, rainy conditions at a heaving King Power Stadium they nearly went ahead.
A terrible defensive slip-up by Manchester United trialist Jeffrey Schlupp allowed Ikechi Anya to run through and Kasper Schmeichel had to be alert to block his strike and an even better follow-up effort from Almen Abdi.
Marco Cassetti also shot over from distance but Leicester remained the dominant side and responded again.
Nugent should have scored but side-footed wide from 12 yards after more great work by the jet-heeled Dyer and Knockaert went close after a lightning break and some smart footwork on the right flank.
The chances slowed up after the interval and the quality and excitement also dropped off. But the frantic pace remained.
Watford began to show the measure of control they had previously lacked and looked more comfortable in defence against Leicester's pace.
And they had two glorious chances to go ahead.
Football League player of the year Vydra should have scored but completely fluffed his kick when through one-on-one and Geijo headed over from six yards at the back post when he seemed to completely mistime his leap.
The Foxes made Watford pay when Knockaert swung over a teasing cross and Nugent rose to head in from close range and send the home fans into raptures and earn a vital lead prior to Sunday's second leg at Vicarage Road.
Leicester City manager Nigel Pearson:
"It's half-time, it was a tight game with two good sides. We were able to take a chance towards the end when it looked as though it was going to end 0-0 and it's all to play for.
"We've got to be positive and try and win the next game as well.
"The play-offs are always a bit difficult to negotiate in terms of anything can happen and both sides will feel they can win."
Watford manager Gianfranco Zola:
"I didn't expect them to score, I didn't see that coming at any time because it looked like we were in control of it.
"We had a few chances when we should have scored.
"But it's only half-time. We are looking forward to playing on Sunday.
"We have to win the game - that is what we want. It is going to be tough but we have a lot of confidence too."