Lloyd Dyer's first goal for Watford earned the Hornets a 1-0 victory at Stevenage and a place in the second round of the Capital One Cup.
The League Two side provided stubborn opposition for their Championship visitors and could have forced extra-time had it not been for a superb save by Jonathan Bond.
Boro almost blasted themselves into a shock lead when Darius Charles held the ball up before laying it off to Charlie Lee, whose wonderful hit from 25 yards came back off a post.
Watford went ahead seven minutes into the second half when Fernando Forestieri played through Diego Fabbrini, who bided his time before rolling it across for Dyer to tap in.
It looked certain that Stevenage would equalise when Charles met Bira Dembele's excellent cross only for Bond to pull off a wonderful stop.
Dyer nearly added a second with eight minutes left when he beat Jon Ashton before hammering his shot against the top of the crossbar.