FA Cup, Manchester United 1-0 Middlesbrough
The otherwise outstanding Middlesbrough defender Jonathan Woodgate brought down Ronaldo from behind as the Portuguese winger cut in on goal along the by-line from the left in the 76th minute.
Ronaldo coolly converted the penalty kick to clinch the tie for United.
Boro keeper Mark Schwarzer twice denied Wayne Rooney in one-on-ones when the England striker was clean through as United applied pressure in their quest to keep alive the dream of another treble.
The closing 15 minutes after the goal became quite heated, with four bookings and a sending off for disgruntled Boro players. But it could not prevent United joining Arsenal on a record number of 25 Sem-Final appearances.
Substitute James Morrison was red-carded for a nasty lunge at match-winner Ronaldo.
Victory was deserved for United, as they should the greater ambition to win. Boro appeared to have come for a draw and penalties - they way they got through the previous two rounds.
But United were not interested in extra time or a shoot-out, and had Boro on the back foot for most of the game. As a result, the visitors' goal attempts were confined to speculative efforts from distance that posed little threat to home keeper Tomasz Kuszcak.
It made for a frustrating evening for Boro's main striker, Mark Viduka, while at the other end Woodgate led by example to repel Ronaldo and Rooney.
Just before half-time, though, Rooney was played through by an astute Ryan Giggs pass, and it took a magnificant save from Schwarzer, diving at his feet, to thwart him.
Middlesbrough's best chance arrived ten minutes into the second-half, when Viduka worked an opening for Stewart Downing, only for the winger to slice the ball wide.
Ronaldo then hit the outside of the post from a tight angle before Schwarzer again enied Rooney, this time with his foot.
However, the Australian keeper stood no chance when Ronaldo sent him the wrong way with a deftly-placed penalty.
The award of the spot-kick seemed straightforward enough - Ronaldo was upended by Woodgate in the England defender's one (costly) error of the night. But it angered Woodgate's team-mates, with George Boateng booked for shoving a ball-boy and Morrison sent packing for his challenge on Ronaldo.
That triggered an ugly melee in which Rooney, already booked, was lucky not to have been further disciplined.
Man Utd: Kuszczak, Brown, Ferdinand, Vidic, Heinze, Ronaldo, Carrick, Giggs, Richardson (Park 60), Smith (O'Shea 71), Rooney.
Subs Not Used: Heaton, Cathcart, Eagles.
Booked: Smith, Rooney.
Goals: Ronaldo 76 pen.
Middlesbrough: Schwarzer, Xavier, Woodgate, Pogatetz, Taylor (Huth 88), Cattermole (Morrison 62), Boateng, Arca (Rochemback 69), Downing, Yakubu, Viduka.
Subs Not Used: Jones, Lee.
Sent Off: Morrison (90).
Booked: Xavier, Woodgate, Pogatetz, Boateng, Rochemback.
