Leicester beat Watford to move five points clear at the top of the table as Riyad Mahrez scored his 15th Premier League goal of the season.
After second-placed Tottenham drew with third-placed Arsenal, the Foxes took a step closer to English football's most unlikely title win.
Mahrez almost set up Jamie Vardy for an early goal as the Foxes started well.
And a similar start to the second half saw the Algerian rifle high into the net after Jose Holebas cleared to him.
The 25-year-old, who has scored or assisted 26 goals this season, worried Leicester after he seemingly pulled his hamstring late in the game and was substituted but Mahrez later said it was only cramp.
Despite good chances for Watford strikers Troy Deeney and Odion Ighalo, the visitors, who flirted with relegation last season, held on and now have nine games between them and a historic first title triumph.
The victory also takes them 11 points clear of fifth-placed West Ham as the Foxes close on a Champions League spot and they are 41 points better off than they were at the same stage last season.
Each week the character and quality of Claudio Ranieri's side is further tested as the pressure is ramped up on the team with the least title-chasing experience.
But since the Foxes drew with West Brom on Tuesday, they have seen rivals Tottenham and Arsenal both beaten, before the two north London sides drew in a fiercely contested derby earlier on Saturday.
Watford did not offer the toughest opposition for the visiting team, but an opportunity to eke out a five-point gap presented its own examination.
And they passed it once again thanks to a solid defensive display, energy from the returning N'Golo Kante, and the skill and accuracy of Mahrez, who proved the difference.
The winger was still able to walk off the pitch after picking up his injury, but any problems for Leicester's star player could yet derail their challenge.
The Hornets suffered a second consecutive 1-0 defeat after their midweek loss at Manchester United, but while they were unlucky not to earn a draw at Old Trafford, here their ambitions were quickly snuffed out by the league leaders.
Foxes centre-back Wes Morgan had a running battle with Ighalo, who missed a close-range header late on, with Nordin Amrabat and Jose Holebas both having shots saved by Kasper Schmeichel.
Nathan Ake also clipped the top of the crossbar with a header but the hosts rarely threatened the Leicester goal.
Ake only moved into central defence after a late injury to Miguel Britos but it looked a stroke of good fortune as his pace twice got Watford out of trouble early on.
Quique Sanchez Flores' team have now failed to score in five of their past six Premier League games and Ighalo has scored only once in 2016.
Promoted last season, they still need one more win to reach the 40-point total that is the usual benchmark for Premier League safety.
Leicester manager Claudio Ranieri: "It was an important win, a big battle. Now there will be only battles for everybody not just for us. A tough match. We created four or five chances, they created two or three. We are so happy now.
"The title is not ours. We have to fight a lot, step by step and keep our feet on the ground. At the end we can see what happens. I'm sorry, we know every team can win or lose so it has to be slowly slowly."
On Riyad Mahrez hamstring: "No injury, he was tired and had a little cramp so I changed him in the late minutes. His goal was so important."
Watford manager Quique Sanchez Flores: "It was very difficult, they defended very well, lots of intensity through the middle. We created some attempts, we make one mistake and when they score first it is very difficult."
On his strikers' goal drought: "Of course. We have problems to score in the last matches. For me it will be interesting not to look at the strikers in this case, it's important to get goals from the other lines, I don't want to put pressure on the strikers."
Leicester have won 31 points away from home this season in the Premier League; five more than any other team.
Eleven of Mahrez's 15 Premier League goals for Leicester City this season have come away from home.
Leicester have scored 33 goals in the second half of Premier league games in this campaign - two more than any other side.
Watford have failed to keep a clean sheet in their last 14 matches in all competitions against the Foxes.
Watford have conceded a league-high proportion of their goals in the second half of matches (73%).
Leicester now have a nine-day break until they host Premier League strugglers Newcastle at home, while Watford face either Arsenal or Hull in the FA Cup quarter-finals next weekend before hosting Stoke in the league the following weekend.
Match ends, Watford 0, Leicester City 1.
Second Half ends, Watford 0, Leicester City 1.
Attempt missed. Danny Simpson (Leicester City) left footed shot from outside the box is high and wide to the left.
Odion Ighalo (Watford) wins a free kick on the left wing.
Attempt missed. Nathan Aké (Watford) left footed shot from outside the box is high and wide to the left.
Substitution, Watford. Mamadou Obbi Oularé replaces Etienne Capoue.
Attempt missed. Jeffrey Schlupp (Leicester City) left footed shot from the left side of the box is close, but misses to the left. Assisted by Daniel Amartey.
Substitution, Leicester City. Daniel Amartey replaces Riyad Mahrez because of an injury.
Attempt missed. Almen Abdi (Watford) right footed shot from outside the box misses to the left.
Substitution, Watford. Ikechi Anya replaces Nyom.
Corner, Watford. Conceded by Riyad Mahrez.
Attempt saved. Odion Ighalo (Watford) header from the centre of the box is saved in the top centre of the goal. Assisted by Troy Deeney.
Attempt missed. Robert Huth (Leicester City) header from the centre of the box is close, but misses to the left. Assisted by Riyad Mahrez with a cross following a corner.
Corner, Leicester City. Conceded by Almen Abdi.
Corner, Watford. Conceded by Robert Huth.
Attempt missed. Robert Huth (Leicester City) header from the centre of the box is close, but misses to the left. Assisted by Christian Fuchs with a cross following a set piece situation.
Attempt saved. Riyad Mahrez (Leicester City) left footed shot from outside the box is saved in the bottom left corner. Assisted by Danny Simpson.
Substitution, Watford. Almen Abdi replaces Mario Suárez.