There is no cloaking the fact the FFA Cup has brought the historical schisms of the game, crudely termed old soccer and new football, into an uncomfortable proximity. Face-to-face with in the in-laws.

No clubs reflect the divide better than Sydney United and Sydney FC, two clubs diametrically opposed in almost every sense. Geographically, historically, financially. But, cliched as it sounds, this is the magic of the cup.

And when United drew level on the brink of half-time, through Stjepan Paric, the home side suddenly believed in the impossible. How they brought the house down: a raw, guttural outpouring, some ten years in the making.

For that moment, Sydney United were as good as the A-League's best. They've always felt that way, if we're honest.

No club in Australia has produced anywhere near as many Socceroos. It took two incredible long-range efforts, both from Chris Naumoff, to see them off. Ali Abbas's third in injury time made the 3-1 scoreline a touch flattering.

United's fans came out in great numbers, too. If they weren't wearing Sydney United gear – and there was plenty of retro, NSL-era jerseys on show – they wore the Croatian checks.

With the Croatian community in Melbourne similarly gathered at Knights Stadium to watch St Albans play Perth, the nationalistic sentiment on the night was as fierce as the coals which grilled the cevapis in the well-patronised canteens.

They didn't miss the opportunity to rip into Sydney's Serbian contingent, either. When Nikola Petkovic bravely volunteered for two corners in the first half in front of the vocal home fans, the reception, to say the least, was colourful. They otherwise chanted relentlessly for the motherland until the 90 minutes were up.

Yet while the atmosphere was edgy, it was never unsafe. Both sets of supporters backed their sides with vigour, not violence, and both mounted claims to being the loudest on the night. A split decibel decision.

It seemed as though the occasion frightened the home side in the opening 20 minutes, repeatedly having to repel dangerous attacks. Corey Gameiro was playing at the point of Sydney's attack but it was it Bernie Ibini, deployed on the right but drifting inboard, who looked the most likely.

However, it was neither of those two who opened the scoring. Naumoff, who saw glimpses of first team action last year, let fly with a cracking, 25-yard effort that seared past United goalkeeper Anthony Costa.

Frustrated, United hit back, literally in the first instance as Chris Triantis went right through Ibini. Gameiro grabbed the arm of referee Kris Griffiths-Jones to protest; he earned a yellow and so did Triantis.

They then made it count on the scoreboard. Perhaps rattled by the jibes from the stands, Petkovic bungled a regulation clearance. His air-swing opened the door for Paric, who squeezed his effort home.

The mood of hope that enveloped Edensor at half-time was palpable. They started to believe in their unlikely band of part-timers.

Unfortunately for the hosts, Naumoff had other ideas, and after being fed by the lively Ali Abbas, he took a touch before firing home a delightful effort from an acute angle.

Not that United would give up. The sting remained to the end. Sydney goalkeeper Vedran Janjetovic, who once called this place home, was given a yellow-card for man-handling Ibrahim Haydar.

The fiery Mirjan Pavlovic, previously of Newcastle and Wellington, wanted some of the action too and before long a melee ensued.

The crowd was loving it, at least until Abbas found himself in the clear, tucking home and sealing the Sky Blues' place in the quarter finals.

The draw for the quarter-finals was also announced on Tuesday night: Sydney FC v Adelaide United, Perth Glory v Melbourne Victory, Palm Beach Sharks v Central Coast Mariners, Bentleigh Greens v Adelaide City.