The fixture was rearranged from Sunday and the torrential rain failed to upset the Reds' flowing football as the team displayed great control, passing and movement on a slippery surface. It took 20 minutes for the visitors to break through as, after Jesse Lingard appealed for a penalty when felled on the edge of the box, Luke Giverin curled home the resulting free-kick in any case.
An incisive move up the field led to John Cofie quickly doubling the advantage with a low shot beyond the reach of Ben Procter in the home goal. A double strike just before half-time emphasised the gulf in quality with Davide Petrucci's terrific through ball despatched with noncholant ease by Will Keane. Cofie then claimed his second by collecting a Larnell Cole pass and rifling home an angled finish.
The second half saw more one-way traffic and Will Keane was on the scoresheet again with a typically well-taken drive in a crowded penalty area on 63 minutes. The Millers managed to put up a brave fight in the second period, despite a number of substitutions, but a couple of late efforts again underlined the Reds' class.
Powerful attacker Cofie sealed his hat-trick with a similar finish to Keane's, finding the net from close range with a low effort following an assist from his strike partner to double his tally for the new campaign.
There was still time for Brazilian substitute Rafael Leao to get in on the act, converting a pass from Marnick Vermijl to ensure Warren Joyce's side racked up their best result of all the friendlies, pipping the 7-1 win recorded at Spartans.