But having held their opponents at arm's length in the intimidating Estadio do Dragao stadium, United progressed 3-2 on aggregate without ever really needing to move out of third gear.
Following last week's 2-2 first leg draw at Old Trafford, United knew they needed to score and Sir Alex Ferguson named an attacking side with Dimitar Berbatov, Wayne Rooney, Ronaldo, Anderson and Ryan Giggs all included from the start.
Rio Ferdinand also returned at the back to sure up a defence which had shipped an alarming amount of goals in recent weeks.
But United's suspect defence was never seriously tested once they had stamped their authority on the tie with Ronaldo's sixth-minute piece of magic.
Anderson fed Ronaldo in the midfield area and after taking a few strides forward, the Portugal winger dispatched a stunning right-foot shot into the back of the net.
Porto keeper Helton dived but got nowhere near the ball as it rocketed past him from fully 40 yards.
United could and should have put the tie to bed in a first half in which they dominated and Porto failed to find the same intensity which characterised their performance at Old Trafford.
Giggs drilled a back-post volley into the body of Helton and Nemanja Vidic was left holding his head in his hands after stabbing a John O'Shea flicked on corner over the top from just three yards.
In-between, Porto sounded a warning with dangerous headers from Bruno Alves which failed to hit the target.
After the interval, Porto offered more as an attacking force but never seriously tested Edwin van der Sar.
Substitute Mariano Gonzalez failed to connect with a header and Hulk lashed a 25-yard free-kick over the wall but straight down the throat of van der Sar on the hour mark.
In the final ten minutes, Lisandro Lopez spurned Porto's best chance of the night when he flicked Mariano's cross straight at van der Sar from eight yards.
United then successfully played out time - helped by a tireless performance from Rooney - to book their place in an all-English semi-final against Arsenal.