Louis Saha had given the visitors a 30th minute lead, before Rovers' flying Norwegian levelled the tie with his seventh of the season.
As the two sides switched focus from one cup competition to another, both managers made a fistful of changes to their starting line-ups.
With a fully fit squad at his disposal, Mark Hughes made five changes to his QPR conquerors - Michael Gray, Ryan Nelsen, David Bentley, Shefki Kuqi and Pedersen all recalled to the team.
Master of mind games, Sir Alex Ferguson, named virtually his strongest side, with Ruud van Nistelrooy and Paul Scholes the only notable absentees.
Playing in front of their biggest attendance of the season, it was the home side who started the game at a blistering pace.
Within the opening 30 seconds, Bentley collected a ball over the top and when he found the unmarked Robbie Savage in the centre, the Welshman looked certain to score his first goal in a Rovers shirt, but his left-footed volley was saved by the legs of Edwin Van der Sar.
On five minutes, Pedersen was given too much time to cross on the left and after Darren Fletcher completely mis-kicked, the unsuspecting Bentley failed to connect with the goal at his mercy.
As the half progressed, United's attacking threat was there for all to see.
Wayne Rooney went close twice and on the half-hour mark the visitors were in front.
Ryan Giggs picked up the loose ball in the centre of the pitch and after striding 30 yards forward, he played the perfect ball through to Saha whose shot across Brad Friedel found the bottom corner of the goal.
But Hughes' men didn't have to wait long to pull level. On 35 minutes Kuqi flicked Nelsen's long ball directly into the path of Pedersen and the wing wizard rifled the ball into the top corner with a stunning right-footed volley.
As the half drew to a close and with temperatures rising, Alan Smith looked a lucky man to stay on the field after a foul on Savage which saw both players along with Rooney receiving yellow cards.
With recent busy schedules over the festive period taking effect, sadly the second 45 minutes never quite lived up to the first.
Van der Sar was never called upon to make a save, watching Kuqi's 12-yard drive go wide, while a series of long-range Rooney strikes were the only things to trouble Friedel's goal.