Reading took advantage of Derby's frail defence to all but secure their place in the Coca-Cola Championship play-offs and leave the Rams looking anxiously over their shoulders.
Second-half goals from Dave Kitson and Sean Long gave the Royals only their second win in 10 Championship games to put them six points ahead of seventh-placed Swansea with just two games remaining, with Steve Coppell's side having a far superior goal difference.
On-loan striker Kitson and striker partner Long also had goals ruled out for offside in the first half, as Derby's disjointed defence struggled to contain them.
Nigel Clough's side have now won only one in nine and are by no means assured of staying up, with third-bottom Norwich five points behind them with two games remaining.
Derby's charitable defending has proved costly in recent weeks and Reading, returning to the ground where they were relegated on the last day of last season, had clearly taken note.
Long had the ball in the back of the net after just two minutes but the referee's assistant flagged for offside.
And two minutes later the lively Long capitalised on some woeful defending and played in Kitson, whose venomous right-foot drive was brilliantly tipped over by Stephen Bywater.
Moments later Reading thought they had taken the lead once again through a bullet header from the flame-haired Kitson, but an offside flag once again came to Derby's rescue.
Although Clough's side looked vulnerable at the back, they were asking questions at the other end.
Luke Varney, in his first appearance since returning from a loan spell at Sheffield Wednesday, shot straight at Marcus Hahnemann from 22 yards out.
Robbie Savage was as combative as ever in the middle of the park and Rob Hulse should have done better than head tamely over the crossbar when the former Wales international picked him out at the back post.
Reading came close once again after 25 minutes when the tricky Jay Tabb ended a mazy run with a cross that seemed to hit Long and flash wide of the near post.
But the Rams started to look increasingly threatening and they came close eight minutes before the break when Kris Commons' cross from the left was deflected narrowly wide by Michael Duberry.
The second half started in a similar fashion, with both sides pressing for the opener.
On-loan winger Glenn Little spurned an excellent chance to break the deadlock after 48 minutes, thumping a right-foot finish wastefully wide despite being unchallenged.
Scotland winger Gary Teale stung the palms of American custodian Hahnemann a minute later but it was Reading who broke the deadlock after 55 minutes.
Mali winger Jimmy Kebe was the architect, swinging in an inviting cross from the left and Kitson got in front of his marker to head beyond the helpless Bywater.
Derby were unable to respond, looking short of ideas going forward and they were left with a mountain to climb when Long doubled Reading's lead after 72 minutes.
The Irish striker broke the offside trap and kept his cool to slot the ball under Bywater.
Commons twice came close for Derby as they tried to salvage something from the game, but Reading were worthy winners.
Teams:
Derby Bywater, Connolly, Albrechtsen, Nyatanga, Todd (Pearson 80), Varney, Savage, Bannan, Teale,Hulse (Villa 83), Commons (Barazite 89).
Subs Not Used: Price, Sterjovski.
Booked: Bannan.
Reading Hahnemann, Rosenior, Bikey (Pearce 87), Duberry, Harding, Little (Stephen Hunt 75), Karacan, Tabb, Kebe (Matejovsky 89), Kitson, Long.
Subs Not Used: Federici, Cisse.
Goals: Kitson 56, Long 73.
Att: 31,345
Ref: Colin Webster (Tyne & Wear).