Cardiff made it three successive wins in the Sky Bet Championship for the first time this season after thrashing Rotherham 5-0 in Wales.
Goals from Kadeem Harris, Junior Hoilett, Craig Noone and a Kenneth Zohore brace gave Bluebirds boss Neil Warnock a deserved victory against his former club.
Warnock took the Millers on an 11-game unbeaten run which saved them from the drop last term.
But now, while he has steered Cardiff up to 12th, relegation to League One is looking more and more of a certainty for Rotherham, with the club 15 points from safety.
Cardiff took the lead in the 11th minute thanks to Harris' third goal in two games. The Millers failed to deal with Aron Gunnarsson's long throw and the ball fell to Harris, who curled his effort past Richard O'Donnell.
Harris, 23, is out of contract in the summer and the Cardiff boss has stated a desire to keep hold of him.
The in-form winger netted twice in Cardiff's 4-3 win at promotion-chasing Derby on Tuesday and has shone under Warnock.
Rotherham nearly hit back with a long throw of their own when Will Vaulks' missile was headed just wide by Richard Wood.
And they were nearly made to pay for that miss as the Bluebirds should have doubled their advantage. Noone's inswinging cross found its way to Harris, whose shot was cleared off the line by Wood after O'Donnell had partially blocked it.
It was a vital clearance from the Millers' skipper, with Joe Ralls close to turning in the rebound.
O'Donnell was keeping Rotherham in the game and had to be at his best once again to deny Zohore from close range just before half-time.
There was a lengthy stoppage when Cardiff's Rhys Healey sustained a serious-looking injury to his left leg and had to be carried off on a stretcher. Hoilett replaced him and scored with his first touch in the third minute of first-half stoppage-time.
The Canada international, who has found first-team chances limited of late, latched on to Ben Purrington's calamitous back pass and kept his cool to give Cardiff a two-goal lead at the break.
There was no letting up in the second half from a Bluebirds side that have now scored nine goals in two games.
Warnock's men added a third strike in the 49th minute after Noone volleyed home Harris' looping cross.
Zohore got in on the act just five minutes later as he blasted the ball in from a tight angle following a neat one-two with Hoilett.
And the Bluebirds talisman netted his second goal just seven minutes from time, expertly lobbing the ball over a helpless O'Donnell.
Rotherham's wait for a first away win of the season goes on, with their last triumph coming at MK Dons in April.