The Hornets need just three more points to guarantee a top-two finish, while Millwall are now jostling for position in the top half.
A 1-0 loss to local rivals Luton Town last week appeared to open the door for the chasing pack to make up some ground on Watford, but that has not proved to be the case.
While others around them continue to stumble, the Hornets returned to winning ways with a 1-0 win at already-promoted Norwich City on Tuesday evening.
That win moves Watford to within five points of top spot, but the more immediate priority for Xisco's side is to seal a top-two finish, which they can achieve on Saturday.
Watford know that a ninth win in 11 league games will be enough to make certain of a return to the Premier League at the first time of asking.
There is also the possibility of the Hornets going up with a draw, but they need Bournemouth and Brentford to draw in the early kickoff for that to happen.
However, a defeat for Watford could yet make things interesting as they face Brentford and Swansea City - two of the three sides directly below them - in their final two games.
Still, this is a match Xisco's men will be confident of winning, having taken all three points from their last seven Championship home games - their best run since 1977.
Millwall have gone off the boil in recent weeks, too, taking just one point from the last nine on offer.
The Lions have had a tough run, losing 3-0 to Swansea City and 4-1 to Bournemouth, either side of a stalemate against another promotion-chasing side in Brentford.
Gary Rowett's side are now aiming to avoid back-to-back losses for the first time this calendar year, though eighth place is now ultimately the best that they can hope for.
Xisco freshened up his side between the loss to Luton and win over Norwich and that paid off as Watford picked up arguably their best win yet.
Jed Wallace scored Millwall's consolation in midweek to make it 10 goals for the campaign, which should see him retain his spot up top.
Watford possible starting lineup:
Bachmann; Femenia, Sierralta, Troost-Ekong, Masina; Zinckernagel, Hughes, Chalobah; Sarr, Pedro, Sema
Millwall possible starting lineup:
Bialkowski; Evans, Pearce, Cooper; McNamara, Kieftenbeld, Woods, Mitchell, Malone; Wallace, Bradshaw
We say: Watford 2-0 Millwall
Watford will know ahead of kickoff exactly what is required to earn promotion - one point or all three.
Either way, this is a game that we expect the Hornets to win against a Millwall side with little left to play for, which is reflected in their recent results.