Tom Bradshaw's double helped Millwall climb into the Championship play-off places with a comfortable 3-0 win over relegation-threatened Rotherham.
The Lions striker took just four minutes to get the hosts off to a New Year flier before Cameron Humphreys' own goal and Bradshaw's late second capped a miserable afternoon for the Millers.
The home side dominated the opening period after Bradshaw's early strike but Rotherham should have levelled on the stroke of half-time when striker Conor Washington inexplicably blazed over an open goal.
Matt Taylor's struggling visitors failed to threaten in the second half as Humphreys' own goal - created by Bradshaw - and the Lions hitman's late second extended the Millers' difficult run to just one win in 12 matches.
Millwall boss Gary Rowett rung the changes from the side held to a 0-0 draw by Bristol City as Callum Styles, George Honeyman and Bartosz Bialkowski all started - with usual first-choice goalkeeper George Long missing out for personal reasons.
And that shake-up almost got the hosts off to the perfect start as captain Shaun Hutchinson headed over from Billy Mitchell's cross.
Millwall took the lead just moments later, however, when Bradshaw rose highest in the Rotherham box to expertly nod in Styles' inviting free-kick.
Rowett's side could have grabbed an almost-immediate second when Andreas Voglsammer was unable to divert Styles' volley on target and Zian Flemming headed straight at goalkeeper Viktor Johansson.
And the dangerous Dutchman was involved once again when his first-time shot from the edge of the box forced Johansson into another straightforward save.
Rowett's side were rampant and should have doubled their lead when Johansson parried Styles' powerful volley and Bradshaw failed to divert the Millwall midfielder's follow-up cross on target.
And the Swedish keeper was called into action twice again at the end of the half as he denied both Honeyman and Flemming's efforts from close-range.
Rotherham finally mustered some form of resistance when Bialkowski repelled Hakeem Odoffin's header after Shane Ferguson's pinpoint cross.
And they missed a golden chance to level at the end of the half when Washington somehow rifled over an empty net after being put clean through on goal by Dan Barlaser and rounding Bialkowski.
Millwall struggled for the same fluency at the start of the second period but came close to netting a second when Johansson made a brilliant reaction save from another Bradshaw header.
The Lions striker nodded narrowly wide just moments later before playing another key role in the hosts' second.
He controlled Flemming's lofted through ball before firing a shot that ricocheted off unfortunate defender Humphreys and into helpless Johansson's net.
Flemming came close to adding a third when he dragged a shot wide from range before Bradshaw hit his second to wrap up the points with just 10 minutes remaining.
Voglsammer drew a strong save from Johansson but Bradshaw was on hand to slot home from close range and fire Millwall up to fourth in the table.