The Londoners had been lethargic before the break when Derby should have taken the lead, but a mistake by goalkeeper Roy Carroll handed the visitors the break they needed.
Once Spurs were ahead, they finished off Derby with Younes Kaboul and Dimitar Berbatov giving the scoresheet a misleading look.
Certainly Derby at least matched Spurs for much of the game, but a lack of quality where it matters has hampered the home side all season and it showed again in front of their biggest crowd of the campaign.
More than 3,000 Spurs fans swelled the Pride Park gate to more than 33,000, but they had little to sing about in the first half when the Rams were the livelier team.
Stephen Pearson's strength and running caused the visitors problems and Giles Barnes should have scored when the midfielder sent him clear in the 24th minute but he shot well wide with the outside of his right boot.
It was a bad miss and Derby probably feared it could prove costly although Spurs showed little in front of goal with Berbatov on the bench.
Darren Bent's first game back from injury ended early in the second half when Berbatov made his entrance and Spurs were a different side with the Bulgarian prompting from the front.
But it was an error by Carroll that proved the turning point when he spilled a low drive from Steed Malbranque at the feet of Robbie Keane who gratefully tucked away his 19th of the season.
Derby felt there should have been an offside flag but replays suggested Malbranque was on when Tom Huddlestone's clever pass found him on the left.
From then on, it was a case of how many Spurs would score as Derby tired and tried to chase the game and only Carroll kept them in the game.
He made a great double save from first Jermaine Jenas and then Berbatov in the 74th minute but was helpless when a corner was knocked down to Kaboul who side-stepped a tackle before firing low into the bottom left corner.
There was more misery for Derby in stoppage time when referee Martin Atkinson ruled that Alan Stubbs had deliberately handled and Berbatov coolly stroked the penalty into the bottom left corner after sending Carroll the wrong way.
By then, another defeat for Derby was sealed and the fans will have noted just how Huddlestone has flourished since he left Pride Park.
Although Spurs missed his creativity in midfield, he was outstanding at centre-back and did not make a mistake in a calm and mature display.
Juande Ramos must be delighted at his progress although he appeared less happy with Michael Dawson after the centre-half left the action early with a hamstring injury.
Derby skipper Robbie Savage was substituted in the second half and the news from the home camp was that he had gone to hospital with a foot injury.