Michael Tonge recovered from illness to leave Watford sweating on their Championship future after another away defeat.
Tonge scored his first goal for two-and-a-half years to put Derby on course for a 2-0 victory that eases their own relegation worries, while Chris Porter doubled his side's lead in the 77th minute as the Rams bounced back from back-to-back defeats to turn in a convincing display.
Derby coach Gary Crosby said: "Michael has been struggling all week with a virus, but he wanted to play for as long as he could."
He added: "Michael adds quality, which he showed with the goal and that was with his supposedly weaker foot. He's very comfortable with possession in any area of the pitch and he is a lovely lad as well.
"He's gone to Stoke and they are doing very very well and he's unfortunate not to be involved there more often than he has been, but that's to our benefit at the moment.
"It was pleasing to get a clean sheet as well, but I'm pleased for the whole team, not just the back four and the goalkeeper."
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