Cristiano Ronaldo was sent off for the third time in his Manchester United career as Wayne Rooney's 100th goal in club football sealed a 1-0 victory for Sir Alex Ferguson's men against Manchester City at Eastlands on Sunday.
Rooney's goal — his first in seven games — settled the 150th meeting between the Manchester neighbours and cemented United in third spot in the Premier League.
But United were forced to play the final twenty-one minutes with ten men after Ronaldo was dismissed for handling the ball from Rooney's corner — his second booking in the space of nine minutes.
With five other players booked, United now face an FA fine for failing to control their players, but Ferguson will not be concerned by that having sealed a crucial victory.
City hadn't landed back in Manchester until early on Friday morning following their UEFA Cup tie against Schalke in Germany, and United went into this game with the advantage of having had an extra two days to prepare.
The extra freshness within the United ranks showed in the first half as the European champions dominated possession and created the best chances.
Not for the first time this season, though, Ferguson's team failed to capitalise on their early dominance and, just as happened at Liverpool, Everton and Arsenal earlier this campaign, United struggled to kill off their bitter rivals.
The visitors could have gone ahead as early as the eighth minute when City keeper Joe Hart fumbled a Rooney shot, but Darren Fletcher failed to convert the loose ball from six yards.
Moments later, Ronaldo went close to opening the scoring when he rose above the City defence to head Rooney's corner inches over the crossbar.
City, who won both derby clashes against United last season, simply could not get into the game and 32 million pounds star signing Robinho was nowhere to be seen as United tightened their grip.
But while the chances continued to be created, United just could not put them away and Dimitar Berbatov's glancing header from Rafael's cross brought a stunning save from Hart.
City finally created a scoring chance on 32 minutes when Javier Garrido's free-kick was met with a weak punch by Edwin van der Sar. The ball dropped to Stephen Ireland on the edge of the penalty area, but the Irish midfielder's shot struck the outside of the post.
Just as the game appeared to be drifting towards a 0-0 half-time scoreline, however, United finally profited from their good work when Rooney pounced to open the scoring.
The home side were unable to clear the danger inside their own penalty area and Michael Carrick's left-foot shot was only blocked by Hart. Hovering yards from the goalline, Rooney was never going to miss.
Although United continued to hold control in the second-half, City at least threatened their opponents' goal on a more regular basis and Benjani Mwaruwari was unlucky not to equalise when he hit the sidenetting from a tight angle on 56 minutes.
Spanish full-back Garrido then sent a free-kick narrowly wide of the United post ten minutes later as Mark Hughes's team attempted to claw themselves back into the game.
United looked lively on the counter-attack, though, and Rooney and Berbatov should have made it 2-0 when they broke forward only to see Richard Dunne clear the danger.
But United were forced into a defensive end to the game following Ronaldo's dismissal for his second yellow card. The Portuguese forward protested, but it was a deliberate handball and referee Howard Webb had no option but to send him off.
City almost scrambled an equaliser in stoppage time when Dunne's shot was cleared off the line by Patrice Evra, although Rooney's long-range chip, well-stopped by Hart, could have put the seal on United's win.