The Rams faced the Bluebirds in a crunch Sky Bet Championship clash in South Wales on Tuesday evening
A Wayne Rooney mistake all but ended Derby’s promotion hopes as Cardiff consolidated their position in the Sky Bet Championship play-offs with a 2-1 home victory.
Jason Knight had earlier cancelled out Junior Hoilett’s first-half effort to restore parity for Derby.
But Derby were punished after 59 minutes when Rooney was caught in possession by Lee Tomlin and England’s record goalscorer made no attempt to retrieve the ball.
Rooney was helpless as Tomlin ran away to restore Cardiff’s lead and keep them in sixth spot and the final play-off place, two points above Millwall.
Cardiff’s play-off destiny is firmly in their own hands with a trip to Middlesbrough – and former manager Neil Warnock – to come and a visit from hapless Hull, who were thrashed 8-0 by Wigan on Tuesday.
Derby, meanwhile, need snookers to make the top six after Rooney’s dreadful error.
The Rams have to win their last two games and hope Cardiff remain pointless, while also hoping that several teams in between somehow fail to pick up enough points.