From a position of seeming invulnerability, Rangers remarkably looked in danger of throwing it all away and survived what looked like a decent shout for a United penalty shortly after Scotland’s goal.
Ref Hugh Dallas waved away the claim and Alex McLeish’s men took full advantage of the let-off to thunder in another five goals to categorically clinch their place in the final on March 20 where they will meet Terry Butcher’s Motherwell who beat Hearts in the other semi-final the previous night.
It was perhaps harsh on Ian McCall’s men who had fought back well from a first-half battering but they could do little when Rangers ended the game intent on scoring with every attack.
As expected, the Light Blue supporters had to wait a while to witness the Second Coming of Barry Ferguson – he started on the bench and came on midway through the second half – although the other deadline day signing, Ronald Waterreus, started in goals.
In truth Ferguson, warming up on the touchline, probably had more to do than the goalkeeper in as one-sided a first half as witnessed in recent times.
Before this evening’s semi-final, United were the only Scottish side not to have suffered defeat from McLeish’s men this season although they offered little resistance when Rangers scored twice and threatened to bag many more in a domineering first period.
With increasing regularity, a long ball lumped forward by either Marvin Andrews or Sotos Kyrgiakos would easily dissect the United central pair of David McCracken and Paul Ritchie who looked entirely uneasy throughout.
Prso, in particular, was enjoying acres of space on the lush Hampden turf and looked in the mood to torture a vulnerable United backline with a wide repertoire of tricks.
The Croatian was the instigator when Rangers went in front after just seven minutes, sliding a ball wide to Thomas Buffel’s whose slide rule pass sent Novo scampering clear of a static defence.
Nick Colgan rushed out of his goal to meet the dashing Spaniard but was too late to stop Novo circumnavigating his prone body before stabbing his shot into the gaping net.
United responded with a Stevie Crawford header that spun high over Waterreus’s crossbar but it was a rare moment of respite for Ian McCall’s men.
With Rangers threatening to cut through them like ribbons in every attack, United were soon two down through another moment of ingenuity from Prso.
There seemed little danger when Andrews thundered a hopeful free kick deep into the penalty box but the former Monaco striker rose imperiously above his marker to direct a header beyond Colgan and in off the far post.
A moment later and Prso should have his second – and Rangers’ third – when he latched onto Alex Rae’s forward pass but, in his attempt to lob Colgan, failed to put enough power in his shot.
It was a glaring opportunity and Prso knew it, beating the turf dramatically like a man who won the lottery then put his ticket through the washing machine.
Rangers continued to bombard United’s goal – Colgan did well to block Kyrgiakos header – and should have ended the match as a contest when they were awarded a penalty six minutes before the half-time interval when Ritchie tugged at Prso’s shirt.
Perhaps overly complacent given the comfort of proceedings at that stage, Novo struck a team effort to endure the first ever spot-kick miss of his career.
It was a let-off for United and, after some harsh half-time words from their manager and with Jason Scotland and Stuart Duff on as substitutes, they looked a side reborn in the second half.
It took Scotland only five minutes to make an impact and he dragged the Dundee side back into the contest when he rifled a low shot past Wattereus when Jim McIntyre’s deflected shot fell kindly into his path.
From a point deep in the first half where they seemed virtually dead and buried, United suddenly sensed Rangers’ vulnerability and went for their throats.
McIntyre was first to threaten with a ferocious left-foot drive that rattled off Wattereus’ right-hand post, dribbled along the goal line, clattered the other post before being eventually cleared.
Just two minutes later and they had a penalty claim rebuked when Crawford’s shot struck Andrews’ outstretched arm and, as Rangers rocked and reeled, Wattereus pulled off his first real save as a Rangers player when he dived to his right to keep out a fierce Mark Kerr drive.
Having weathered the storm, it was Rangers’ turn to plant the sucker punch when they extended their lead and extinguished United’s threat in the process.
For the umpteenth time, Novo made his way to the by-line and when he cut the ball back Buffel was waiting to stab in his first Rangers goal with 67 minutes gone.
That set the stage for the belated return of Ferguson and he took the field to an enormous roar from the Rangers supporters packed around two-thirds of the stadium.
Five minutes later Steven Thompson also emerged from the substitutes bench and the towering striker had a major role to play when the Light Blues netted twice more to put an extra sheen on the scoreline.
After 77 minutes Thompson was jostled to the ground by Stuart Duff and Fernando Ricksen stepped up to curve an emphatic free kick that left Colgan grasping at thin air as the ball nestled in the back of the net.
Three minutes later and Thompson was this time the executor as he cottoned on to Mo Ross’ searching cross to thump a half-volley past the helpless keeper.
The scoring didn’t end there, however, and with six minutes remaining Novo made up for his earlier miss by battering Ferguson’s quickly free kick into the corner of the net before Thompson stabbed home his second of the game – and Rangers’ seventh – in the closing seconds.
RANGERS (4-4-2): 1 Wattereus; 3 Ross, 4 Kyrgiakos, 5 Andrews, 6 Ball; 7 Buffel, 2 Ricksen, 8 Rae (Ferguson 70), 11 Vignal; 9 Prso (Thompson 75), 10 Novo
Subs not used: Malcolm, Namouchi, G Smith
Booked: None
DUNDEE UNITED (4-4-2): 1 Colgan; 2 Mair (Dodds 75), 6 McCracken, 5 Ritchie, 3 Archibald (Duff 46); 8 Wilson, 7 Kerr, 4 McInnes (Scotland 46), 11 Robson; 10 McIntyre, 9 Crawford
Subs not used: Grady, Bullock
Booked: Kerr (13), Archibald (38)
Man of the Match: Dado Prso