Skipper Sam Morsy scored the winning goal as Ipswich returned to the top of the Championship for the first time since August by beating Watford at Vicarage Road.
The victory put them two points clear of Leicester - who play Millwall on Wednesday - and enabled them to reach 50 points before the halfway point of the season in their bid for back-to-back promotions.
Watford went ahead when Yaser Asprilla punished an error by goalkeeper Vaclav Hladky, but George Hirst brought Town level before half-time from close range.
It looked like a draw was on the cards, but Morsy dispossessed Wesley Hoedt and slipped the ball past keeper Ben Hamer to secure Ipswich's fourth successive win - and seventh away from home this season.