Robbie Keane's 20th goal of the season kickstarted a hard-fought Tottenham win at Pride Park to continue Derby's woeful form in the Premier League.
Robbie Keane and Jamie O'Hara celebrate the Irishman's opener
It was hardly one-way traffic against a battling Derby, who should have taken the lead when Giles Barnes fired wide.
But Keane, who had earlier hit a post, settled Spurs' nerves after Roy Carroll had spilled Steed Malbranque's shot.
Younes Kaboul made it two with a drive and Dimitar Berbatov wrapped it up from the spot after Alan Stubbs's handball.
The result means the Rams need nothing short of a miracle to maintain their Premier League status next season, though manager Paul Jewell will have been encouraged by an improved display.
The two teams could hardly have come into the match in more different form, with Derby yet to win under Jewell in 14 league and cup games and Spurs rejuvenated under Juande Ramos.
But there was little to choose between the sides in a close first half, with Derby's hassle and harry ensuring they more than held their own against the Londoners.
In fact, had Barnes managed to find the target from Stephen Pearson's through-ball with just the keeper to beat on 25 minutes, it could have been even better for the Rams.
Emmanuel Villa was also lively, forcing Spurs keeper Radek Cerny into a smart stop at his near post, but most of Derby's best work came through the tireless Kenny Miller.
Spurs were not helped by an early injury to Michael Dawson, but they still showed enough glimpses of their attacking guile to keep Carroll on his toes on the keeper's home debut.
Aaron Lennon tormented the Derby defence and he was the creator of the visitors' best early chances, Carroll saving well first from Darren Bent's stinging drive and then Keane's clipped shot.
It was a similar story in the second half and it was not long before Ramos introduced Berbatov from the bench in an attempt to find the cutting edge.
Jenas and Savage enjoyed a fine battle in midfield at Pride Park
Derby, who had conceded 52 goals in the league before the start of play, were hardly airtight at the back, Tom Huddlestone forcing another good save from Carroll.
But the hosts grew in confidence as the game went on and Miller and Barnes were still causing Spurs the odd nervous moment at the back.
However, minutes after Keane had hit the post with a swiped shot from 20 yards, the Irishman gave Spurs the lead, turning home from close range after Carroll failed to hold Malbranque's shot.
It was heartbreaking for the Rams but a real shot in the arm for the visitors, who could have doubled their lead but for a brilliant double save from Carroll, first from Jenas and then Berbatov.
The visitors were now rampant and Kaboul doubled their lead after skipping round Stephen Pearson and driving home.
There was still time for Berbatov to give the scoreline a rather harsh look from the penalty spot after Stubbs had handled Keane's cross as Spurs took their recent run to one defeat in seven matches.
Derby boss Paul Jewell:
"For an hour we did very well and we created chances and so did they. We rattled them at times and they are a good side.
"Our lads could not have given any more but we got punished for basic errors.
"It is a harsh lesson but it is a harsh league and we are not good enough."
Tottenham assistant manager Gus Poyet:
"It was tricky, especially in the opening hour, but the players are learning what we want from them and in the last half an hour we were fresher and took control.
"It was a big test for us - more than people probably think - and it was important that we came here and won because we want to move forward and be consistent in the league.
"We have three massive cup games coming up now and it was vital we signed off with a league win so we're delighted."
Derby: Carroll, Leacock, Stubbs, Moore (Todd 11), Edworthy (Jones 78), Fagan, Savage (Sterjovski 74), Pearson, Barnes, Villa, Miller.
Subs Not Used: Price, Earnshaw.
Booked: Leacock, Miller, Stubbs, Jones.
Tottenham: Cerny, Hutton, Dawson (Kaboul 14), Huddlestone, Chimbonda, Lennon, Jenas, Malbranque, Boateng (O'Hara 46), Keane, Bent (Berbatov 58).
Subs Not Used: Robinson, Gunter.
Goals: Keane 68, Kaboul 81, Berbatov 90 pen.
Ref: Martin Atkinson (W Yorkshire).
BBC Sport Player Rater man of the match: Tottenham's Robbie Keane 7.89 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.