Watford's victory pushed Derby deeper into trouble, but Vicarage Road manager Ray Lewington, although relieved, warned there are many twists and turns ahead.
"This was a must win game and we won it and won it quite well. All the other results appeared to go our way and so we have moved up the table and put pressure on others," he said.
"But we could lose on Saturday and all the rest win and the situation could be reversed. We need three more wins and I think that will do it." Rams boss George Burley admitted that Watford were the better side on the night.
"They hit the ball early and played the conditions better than we did. They proved too strong for our defence which was disappointing," he said.
"We have a game in hand and Watford have a very hard run-in." Lee Grant, the Derby keeper who was born just up the road from Watford's ground, excelled on the night and preventing the Hornets from chalking up a bigger victory.
But it had looked promising for Derby when they took the lead after ten minutes. Candido Costa climbed at the far post to head a cross back into the middle and although Watford's Jerel Ifil should have cleared he mishit the ball and sliced it straight to Paul Peschisolido, who gratefully accepted the chance.
Heidar Helguson had a header tipped away by the keeper soon after this and Watford's insistence paid dividends after 19 minutes. Lee Cook turned to send a shot against the near post and it clipped straight across the goal to Helguson who turned home the equaliser.
Watford continued to look the better side and Scott Fitzgerald headed over when presented with a good chance just before the interval.
But Watford maintained the momentum in the second half, taking the lead after 53 minutes. Helguson headed Neal Ardley's pass into the path of Gavin Mahon, who went through to beat Grant with a well-struck shot.
Twice Helguson came close to increasing the lead only to be denied by Grant, who also went full length to push wide a shot from Fitzgerald.
Peschisolido had one second-half chance when he escaped Ifil, but his angled shot was read by Lenny Pidgeley, who made a comfortable save.