A place at Wembley next month in the reward for the winning side in this final tie of the weekend, and both teams head into the match on the back of vital victories last time out.
Despite bouncing between the second and third tiers of English football over the past 15 years, Millwall are competing in their fourth FA Cup quarter-final during that time. To give that some context, seven-time winners Liverpool have reached three.
The Lions have an incredible love affair with the most famous club cup competition of them all, and they certainly know how to pull off a big upset.
Millwall followed that up with a scrappy 1-0 win at fellow underdogs AFC Wimbledon in the last round, avoiding defeat on their travels in the FA Cup for the first time in five attempts.
It is at home, in front of their vociferous supporters, that Neil Harris's side really come to life, winning their last five such matches against Premier League opponents since losing to Bolton Wanderers in 2012.
Everton are not the only top-flight club to have come unstuck against Millwall in recent years, then, as the Lions have won four FA Cup ties against the so-called big boys since the start of 2016-17 alone - more than any other Premier League side has managed in that time.
Harris was part of the Millwall squad that made it all the way to the final in 2003, where they were soundly beaten 3-0 by Manchester United in Cardiff. Including that season's magical run, the South London side have reached the quarter-finals 11 times as a non top-flight team, a return that only Southampton (13) can better.
Their league form may be patchy, with their four-game losing run ending last time out courtesy of a 2-0 victory at Birmingham City, but that counts for little when it comes to Millwall and the FA Cup.
Recent form in FA Cup: WWW
Recent form (all competitions): WLLLLW
Brighton & Hove Albion
After recording back-to-back Premier League wins over the past fortnight, Brighton can now afford to focus solely on the FA Cup as they target a first major cup triumph in their history.
Brighton have progressed from just one of those previous quarter-final ties, in fact - in the 1982-83 campaign when losing 4-0 to Man United in a final replay, having drawn 2-2 in the first match.
A rare trip down Wembley Way is now there for the taking for Chris Hughton and his players, but if they are to book a place in the semi-finals they must win a third consecutive away FA Cup match for the first time since December 1932.
Hughton's side were 3-1 winners at Bournemouth in the third round, before overcoming West Bromwich Albion by the same scoreline after extra time in a fourth-round replay, following a stalemate across the opening 90 minutes.
Then, despite Hughton's decision to make eight changes with Premier League survival taking priority, the south coast side saw off Derby County 2-1 at the Amex Stadium last month to set up this winnable quarter-final tie.
Thanks to those recent victories over Huddersfield Town and rivals Crystal Palace, Albion are now five points clear of the relegation zone and all but assured of safety judging by the form of the sides below them.
This season may well be defined by Sunday's result, then, as Hughton has a chance to guide Brighton into the semi-finals of the FA Cup for just the second time in their history. All they have to do is avoid slipping up against a side that has made a habit out of bloodying the noses of top-flight visitors.
Recent form in FA Cup: WDWW
Recent form (all competitions): WLWLWW
Conor McLaughlin is Millwall's only confirmed injury absentee, so Harris has near enough a fully-fit squad to choose from.
The Lions have also been without Wales international striker Tom Bradshaw since November, however, and this match is likely to come too soon for his comeback.
Harris made four changes for the morale-boosting win at Birmingham, bringing in David Martin for Jordan Archer in goal, while Ryan Tunnicliffe, Ryan Leonard and Ben Marshall replaced James Meredith, Shaun Williams and Tom Elliott, likely with one eye on this Brighton clash.
Hughton has heavily rotated in the previous rounds but he will go with a much stronger starting XI this weekend, with a trip to Wembley at stake.
Anthony Knockaert is among those in contention to start, and the French winger has been involved in four goals in his last five FA Cup games for Brighton (two goals, two assists).
Millwall possible starting lineup:
Archer; Romeo, Pearce, Cooper, M.Wallace; J.Wallace, Williams, Leonard, Ferguson; O'Brien, Gregory
Brighton & Hove Albion possible starting lineup:
Button; Bruno, Burn, Duffy, Bong; Knockaert, Propper, Stephens, March; Jahanbaksh; Murray
Head To Head
Millwall and Brighton have not met in the FA Cup since the 1956-57 season, with the Lions winning a replay to progress through to the second round.
The Seagulls have not lost any of their last six visits to The Den, all in league competition between 2009 and 2015, winning three and drawing three.
March 2015's Championship match, the last time these sides faced off, ended in an uninspired goalless draw at The Den.
We say: Millwall 1-2 Brighton & Hove Albion
Millwall may be the lowest-ranked side left in this season's FA Cup, but they have a good home record against top-flight clubs in this competition. Both managers can be forgiven for focusing solely on a place at Wembley, even if their sides are struggling in terms of league position, and if Albion name a strong side then we are backing them to come away with a narrow victory.