Chris Wood's 10th-minute goal separates the two sides at the half-time break in the Championship clash at Vicarage Road.
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Wood charged down a Manuel Almunia clearance and the ball ricocheted off the 21-year-old and into the net to give the away side a 1-0 lead.
The New Zealand international thought he had doubled his tally in the 28th minute when he converted from Anthony Knockaert's deflected shot, but the striker had strayed into an offside position.
Wood continued to cause problems throughout the first half and had an appeal for a penalty turned down in the 34th minute after being bundled over by Gabriele Angella.
Polish defender Marcin Wasilewski headed narrowly over the bar in the 43rd minute as Leicester continued to dominate proceedings.