Mileta Rajovic scored twice as in-form Watford continued their fine record against Rotherham with a convincing win at Vicarage Road.
The Dane turned in two left-wing crosses by Ken Sema before the break while Edo Kayembe fired his first goal for the club after the turnaround and Tom Ince and Matheus Martins made it 5-0 late-on.
The Hornets are up to 13th and unbeaten in six, their best league run since April 2021, and have conceded just three goals in that spell.
The third-bottom Millers' winless away run is now 21 league games spanning more than a year, while they have taken just a point from their eight road games this season.
Valerian Ismael made five changes for Watford with keeper Daniel Bachmann replaced by Ben Hamer after a concussion in training. Francisco Sierralta (thigh) and Jeremy Ngakia (illness) also missed out with Giorgi Chakvetadze and Vakoun Bayo dropping to the bench and Ryan Porteous, Ismael Kone, Ryan Andrews and Rajovic recalled.
Jordan Hugill, Lee Peltier, Sean Morrison and Seb Revan were all handed starts by Millers boss Matt Taylor after their dramatic midweek draw against Ipswich. Daniel Ayala, Cafu, Fred Onyedinma and Hakeem Odofin were among the subs.
It took just nine minutes for Watford to take the lead when Sema's curling cross from the left was poked home by the stretching Rajovic.
Sema saw his shot saved by fellow Swede Viktor Johansson before the Millers were forced into an early change with Cafu brought on for Sam Clucas just after the half-hour.
Cafu stung Hamer's palms with a dipping 25-yard free-kick moments later, but Watford's second goal arrived in first-half stoppage-time when Wesley Hoedt freed Sema down the left once again and his low cross was smashed high into the net by Rajovic for his sixth goal of the season.
After the break Oli Rathbone drove wide of Hamer's upright but it was 3-0 on 54 minutes when Porteous fed Kayembe to spin and fire into the far corner.
Rotherham were ran ragged and finished with 10 after Cafu limped off once all their subs had been made, with Watford adding salt to the wound on the break with Ince slotting home from a low Andrews cross in the dying stages.
Rhys Healey then teed-up Martins to fire the fifth goal from the edge of the box deep into stoppage time.
Watford boss Valerien Ismael told BBC Three Counties Radio:
"The first two goals were exactly what we wanted, to come around the block of the defence. We scored in the right moments of the game, early and just before half-time, which opened the game up.
"We made a lot of changes and today was a team performance - the bench gave us another wave. Five goals and another clean sheet just before the international break.
"We've got 12 points from a possible 18 and six games unbeaten so it will give us a lot of confidence.
"We've learned from last week and from what we've done in training every day this week - that's the most pleasing thing."
Rotherham manager Matt Taylor told BBC Radio Sheffield:
"It was a strange first half. We were in the game but nowhere near it in terms of the goals we have conceded. Individual poor moments have really cost us. You find yourselves 2-0 down and you have to chase the game.
"The second half unravelled into such a difficult period with the injuries and the goals against us. There was naivety, mistakes were made.
"We've not defended away from home at any stage this season. We were in the game in the first half but the ease and the manner those goals went in is unforgivable.
"My worry is that's a group of players who aren't believing."