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Watford controlled the game's opening moments, and took an early lead after just four minutes when Allan Nyom picked out Ighalo from the right, before the Nigeria international finished past Sunderland goalkeeper Costel Pantilimon.
The Hornets missed two good chances to extend their lead as Pantilimon kept out an Almen Abdi effort, while Ighalo tried his luck in a difficult position, only for the former Manchester City man to deny the striker in the bottom corner.
Sunderland striker Fabio Borini spurned two chances to net a first-half equaliser, as the former Liverpool man fired wide from a tight angle, before missing the target from a corner.
Rodwell tried his luck from distance again moments later, but the former Everton man's effort went high and wide, before the midfielder was picked out in the box by Duncan Watmore, only to miss the target.
Sunderland almost found an equaliser when Watmore fired narrowly wide from 25 yards, while both keepers were called into action as Pantilimon denied Ighalo, before Gomes kept out a Defoe effort at the other end of the pitch.
Ighalo could have wrapped up the victory with a second goal before the final whistle, as Troy Deeney led a swift counter attack before picking out Ighalo, only for the goalscorer to be denied by Pantilimon.