John O'Shea celebrates after scoring his first goal of the season
Manchester United booked their place in the Carling Cup final despite being given a late scare by Derby County.
The Rams led 1-0 from the first leg but a fierce Nani strike levelled the tie and defensive errors let John O'Shea and Carlos Tevez put United in charge.
Derby improved after the break and Giles Barnes replied from the spot when Paul Green was fouled in the area.
Cristiano Ronaldo's penalty seemed to have sealed United's win but Barnes' superb free-kick set up a tense finale.
It is United who have reached Wembley, where they will take on Burnley or Tottenham, but the Championship strugglers still deserve praise for the spirit they showed at Old Trafford when the game looked dead and buried.
It also emphasised how there could have been a very different outcome had Derby made the most of their opportunities in the first leg.
The Rams had won that game courtesy of Kris Commons' strike but they wasted several other chances and their failure to take more than a one-goal cushion to Old Trafford was always going to make their task in the second leg a difficult one.
Things began promisingly enough for Derby when Commons hustled Darron Gibson off the ball and fired in a rasping shot that United keeper Ben Foster was grateful to see fly wide.
But, once United settled and began to probe the Rams defence, they found gaping holes and took full advantage.
On 16 minutes, Nani was allowed to pick up the ball on the half-way line and advance unchallenged towards the penalty area before letting fly with a ferocious shot that Roy Carroll got a hand to but could not keep out of his net.
Things got worse for Derby six minutes later when Gary Teale stepped up to try to catch O'Shea offside but only succeeded in giving the Republic of Ireland defender time to pick his spot for his first goal this season.
And their defence disintegrated altogether on 34 minutes when Giggs fed Rafael with a quick corner and the Brazilian whipped in a curling cross that gave the unmarked Tevez the easy task of heading home from close range.
The tie looked done and dusted at that point and United, understandably, took their foot off the pedal after the break.
Carroll had to claw away a free-kick by Giggs, who had been in vintage form early on, immediately after the restart but from that point onwards it was all about Derby.
The Premier League leaders were not exactly under pressure but they were no longer threatening, while the Rams went close with Rob Hulse heading over and Green missing the target with a volley.
Things got even more interesting when defender Jonny Evans picked up an injury with 15 minutes left but was forced to play on as United had used up all three of their substitutes.
Evans was clearly struggling and that was a factor when he clipped Green in the box with 10 minutes left, allowing Barnes to drill home from the spot.
United responded well and Ronaldo, who had come off the bench, appeared to wrap things up with a penalty of his own after Tevez was brought down by Carroll.
But Barnes had other ideas and his superb free-kick in injury-time brought Derby within one goal of forcing extra-time.
It was not to be but the night was still far less straightforward than United might have anticipated at the interval.
To add to United's problems, Rafael and Anderson also picked up injuries and could face long lay-offs at a crucial stage of the season.
Man Utd: Foster, Rafael Da Silva (Fletcher 42), Neville (Chester 67), Evans, O'Shea, Nani, Gibson, Anderson, Giggs (Ronaldo 58), Welbeck, Tevez.
Subs Not Used: Kuszczak, Tosic, Scholes, Possebon.
Booked: Rafael Da Silva, Fletcher.
Goals: Nani 16, O'Shea 22, Tevez 34, Ronaldo 89 pen.
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