Jose Mourinho did not do Chelsea much good as Man United turned the heat up under Luiz Felipe Scolari with a thumping win.
Chelsea suffered their worst Old Trafford defeat in 15 years as United eased to a thoroughly-deserved success after Nemanja Vidic had broken the deadlock in first-half stoppage time.
Wayne Rooney added another just after the hour mark before Dimitar Berbatov completed victory for Sir Alex Ferguson's men near the end.
With Mourinho looking down from the stands in judgement on his old club, this would have been the perfect time for Scolari to start proving his doubters wrong.
The decision not to start with Nicolas Anelka, especially with Rio Ferdinand's back injury ruling him out, seemed strange.
But to start with Chelsea seemed more incisive in midfield, even if their hosts had more possession.
Frank Lampard and Michael Ballack both had chances and a more natural offensive player than Ashley Cole would surely have profited from Deco's inspired pass instead of allowing it to bounce through to Edwin van der Sar.
In return a viciously curling Giggs free-kick caused Petr Cech a problem, more so than Berbatov's scuffed shot after being invited to shoot by Rooney.
But, with Cristiano Ronaldo and Ballack also having penalty claims turned down, a hotly-contested scrap was still waiting for its switch to be triggered as it entered the last couple of first-half minutes.
Ronaldo might have flicked it but he mis-hit a shot that bounced into Rooney. Then Darren Fletcher had a penalty appeal turned down when his effort struck John Terry on the arm.
Instead, United got the corner which will still be debated years from now.
Rooney seemed to know exactly what he was doing when he rolled the ball into, then out of the quadrant. Giggs ambled over, crossed and Ronaldo - expected to be crowned FIFA world player of the year in Zurich on Monday - powered home a near-post header.
Referee Howard Webb and his linesman said no. United reacted with fury at the perceived injustice of it all, surrounding the linesman who had made the decision. Chelsea were merely bemused.
Amazingly, Chelsea were still trying to take it in as Giggs swung over the re-take. This time Berbatov got there first, flicking the ball on towards the far post where Vidic arrived to power home.
A month ago, Vidic scored United's injury-time winner against Sunderland before getting sent off in the FIFA Club World Cup final win in Japan that might yet rule him out of the Champions League clashes with Mourinho's Inter Milan.
This was something different and prompted Scolari into a half-time reshuffle, with Anelka replacing Deco.
The move worked to the extent that Chelsea were able to enjoy more sustained spells with the ball.
However, the double negative for the visitors was that their finishing was poor and their need to attack played to United's strengths.
Nevertheless, Chelsea's goal should not have been under threat once Patrice Evra exchanged passes with Ronaldo but then skimmed Berbatov's head with the left-wing cross.
Cole seemed to be in control of the situation but Rooney had other ideas as he pushed his leg between the England full-back's and got enough power into a shot to divert it past a startled Cech.
And when Ronaldo picked out Berbatov with a low free-kick four minutes from time that gave the Bulgarian an easy finish, Scolari could probably feel Mourinho's eyes burning into him from above.
But this is a different team to the one Mourinho crafted. Some of the names may be the same but their power has waned.
Once the most feared Premier League striker, Didier Drogba hit one shot so wide he conceded a throw-in, another skidded off the side of his foot and missed the target by 20 yards.
None of this will worry United, of course. They are now a single point adrift of the Londoners with two matches in hand. And if they win the first of them, against Wigan on Wednesday, they will only trail Liverpool by two.
Given those statistics, it is maybe little wonder Rafael Benitez went for Ferguson's jugular earlier in the week. It might be the best chance he gets.
Man Utd Van der Sar, Neville, Vidic, Evans, Evra (O'Shea 66),Ronaldo, Fletcher, Giggs (Carrick 79), Park, Berbatov, Rooney.
Subs Not Used: Kuszczak, Anderson, Scholes, Welbeck, Tevez.
Booked: Ronaldo, Rooney, Park.
Goals: Vidic 45, Rooney 63, Berbatov 87.
Chelsea Cech, Bosingwa (Belletti 64), Carvalho, Terry,Ashley Cole, Mikel, Joe Cole (Di Santo 85), Lampard, Ballack,Deco (Anelka 46), Drogba.
Subs Not Used: Cudicini, Ivanovic, Ferreira, Kalou.
Booked: Lampard, Bosingwa, Carvalho, Terry, Belletti.
Ref: Howard Webb (S Yorkshire).