Almen Abdi's second-half strike gave 10-man Watford three Championship points away at Reading in Steve Clarke's first game as Royals manager.
Abdi arrived in the box to score from Ikechi Anya's cross, after Gabriele Angella was sent off just before half time.
Although the hosts made a good start, it was Watford who had the first real chance and Adam Federici was forced into a smart save to deny a Daniel Pudil header from an Adlene Guedioura cross.
Federici was forced into more action a few minutes later when Matej Vydra and Troy Deeney combined to set up Almen Abdi, whose powerful shot was beaten away.
The home side went closest towards the end of the half when Chris Gunter scampered down the right and crossed for Nick Blackman, but he headed the ball over the bar at the back post.
Glenn Murray almost made Watford pay for Angella's red card shortly after the break as he latched onto Jordan Obita's pass in the channel, but fired wide after cutting back inside before Garath McCleary stabbed Simon Cox's cross wide with the goal open.
McCleary then turned provider for Blackman as he clipped a ball to the back post, but the unmarked striker sliced the ball wide from six yards.
However, the visitors struck 20 minutes from time as Abdi drilled the ball in from Anya's cross, after Guedioura had won possession in midfield, to keep the Hornets in the playoff positions.
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