Wanderers, who started Saturday's match at Bankwest Stadium in eighth spot, one place behind United on goal difference, scored two in the first half and three more in the second.

They knocked Brisbane out of sixth place, but the Roar have three games in hand and are only a point behind.

United coach Mark Rudan said it was a tired and lethargic performance from his side, but he didn't blame his players who were backing up after a midweek match and had to fly on the day of the game.

It was the Wanderers' most complete and impressive performance of an inconsistent campaign.

"I thought we showed probably for the first time a 90-minute performance," Wanderers' coach Carl Robinson said.

Troisi opened the scoring with a fine 19th minute solo effort and was involved in the build up for the next three goals.

"Jimmy, I thought had his best game (for us), he was industrious," Robinson said.

"Some times the final-third product - and that's what attacking players get judged on, whether its goals or assists - have been slightly off even though he's created things and scored one or two goals.

"But I thought today he was excellent from the start of the game until I took him off, a very, very mature performance by him."

Troisi trigged the landslide when he ran from close to halfway and lashed a fierce shot past United goalkeeper Ryan Scott from just outside the area.

Defender Ziggy Gordon doubled their lead from a 36th minute Troisi corner to reward Wanderers for a dominant first half, in which they had 14 shots to one and the only six attempts on target.

Western Sydney piled on the agony for the visitors in the second half.

Bruce Kamau turned the ball home in the 57th minute and United substitute Dylan Pierias scored an own goal nine minutes later.

Substitute Bernie Ibini blasted the ball home from close range just before the end to cap off a wonderful night for the Wanderers, while United conceded five for the first time in their history and suffered their largest-ever loss.

United, whose talisman Alessandro Diamanti came off the bench In the second half, had more chances than in the first 45 minutes, but good goalkeeping from Scott prevented them conceding more in the first half.

"I feel for my players big time having to fly on the day of the game against a team that's had seven days to rest and recover at home," Rudan said.

"We're backing up after three days on a very soggy and heavy pitch in Brisbane and that's what I put it down to, I just feel extremely sorry for my players.

"I can't fault them at all.

"If anything I feel for them for what they've had to endure, what they've had to go through. It's not normal, it's not fair."

Australian Associated Press