Gianfranco Zola praised Watford's resolve after they capitalised on Hull City's defeat at Bolton Wanderers to move into the Championship's automatic promotion places.
The Hornets took the lead when top scorer Matej Vydra tucked away his 20th league goal of the season in fine fashion from 18 yards after racing clear of the Rams defence.
A third successive Watford win looked like it had been assured midway through the second period when Joel Ekstrand bundled in a Almen Abdi corner.
But Jamie Ward gave Derby hope soon after, blasting in a penalty which had been awarded for an apparent push following a corner.
The Hornets head coach said: ``Obviously if you ask my opinion about the penalty, I'm not very happy about it. I thought it was a soft one. If that was a penalty, how many penalties do you have to give in the box?
``But okay, the penalty was given, no complaints. I think it's made it more difficult for my heart but we held on anyway and we came out with three points that are very, very important, especially because we were playing against a good team and it was the third game in a week.
``The third game in a week is always tough, especially for teams like us who did very well in the first two. If you look around you can see the others had problems.''
Asked if his players had got a little nervous after conceding the penalty, Zola responded: ``I think we were getting tired obviously. I think there were eight players out of 11 that played 90 minutes today, 90 minutes Tuesday, 90 minutes the other Saturday. When you are tired, things become more difficult. The only nervous one was me.''
The defeat sees Derby fall further away from the play-off places, leaving them eight points adrift of sixth-placed Brighton having played a game more.
But boss Nigel Clough was not too disappointed with his side's efforts.
``We're quite pleased with the performance generally today and it's little things in games, either a bit of quality - with the finish in the first half and we had a situation better than that with Michael Jacobs and he failed to get a shot off,'' he said.
``We had numerous situations today and it's been a failing of ours, we don't make the most of them in the final third. Whether it's decision making or a lack of quality and then our old Achilles heel, failing to defend a corner as well has ultimately cost us the game today.
``There was nothing in the game generally between the two teams. They were two passing sides, I didn't think there was a great deal, just that little bit of quality and us failing to defend the set play. Very simple things win or lose football matches most of the times, basics.''
The manager could not speak highly enough of side's work-rate and added: ``They're absolutely knackered again, they give it absolutely everything. I just said to them you've got to learn that you're wasting your own work in some ways.
``You're putting so much in and there's just little things that are stopping us getting a point or three.
``The second we rectify those or do whatever we can, then they'll be getting the rewards for their hard work. You can't fault their effort at all.''