Manchester United manager Jose Mourinho masterminded a brilliant and unexpected 2-0 home victory over former club Chelsea on Sunday to breathe new life into the Premier League title race.
Mourinho gambled by resting Zlatan Ibrahimovic, but Marcus Rashford and Ander Herrera's goals gave United a win that leaves Chelsea just four points above Tottenham Hotspur with six games to play.
Victory also lifted United to within four points of fourth-place Manchester City, on whom they have a game in hand, ahead of the derby at the Etihad Stadium on April 27.
It was a personal triumph for Mourinho, who saw his side thrashed 4-0 at Chelsea in October and was branded "Judas" by fans of his old club during last month's FA Cup defeat at Stamford Bridge.
A wet, cold Manchester afternoon was the setting for United to end a run of 12 games without victory against Chelsea, stretching back to a 3-2 away win in October 2012.
Chelsea have now lost twice in four games, having gone down 2-1 at home to Crystal Palace earlier this month, which will give in-form Spurs fresh hope that the Blues may yet collapse as the finishing line nears.
The top two meet at Wembley in the FA Cup semi-finals next weekend in what now looks a significant test of Chelsea's mettle.
United, meanwhile, will approach Thursday's Europa League quarter-final second leg at home to Anderlecht, which is poised at 1-1, on the back of their best display under Mourinho.
Chelsea were without goalkeeper Thibaut Courtois due to an ankle problem, while Marcos Alonso succumbed to injury in the warm-up at Old Trafford, bringing Asmir Begovic and Kurt Zouma into their usual first XI.
Mourinho set United up to stop Chelsea, with Herrera and Matteo Darmian tasked with the responsibility of tracking dangermen Eden Hazard and Pedro wherever they went.
But despite Herrera's chief role as a stifler, he created United's seventh-minute opener, albeit in contentious circumstances.
He seemed to block Nemanja Matic's pass with his right hand, but referee Robert Madley played on, enabling Herrera to split the Chelsea defence with a sublime through ball for Rashford.
- Rashford ovation -
The 19-year-old raced in behind David Luiz before clipping a shot past Begovic for his ninth goal of the season.
With Chelsea striker Diego Costa spending more time provoking his opponents than taking shots at goal, David de Gea was a virtual spectator between the posts for United.
Costa became embroiled in penalty-box tangles with United centre-backs Eric Bailly and Marcos Rojo and was booked for a late challenge on Paul Pogba.
With the willing Jesse Lingard playing in support of Rashford, United looked far more lively going forward.
Ashley Young twice spurned presentable shooting opportunities from the edge of the box, while Rashford saw a cross headed onto the roof of the net by Gary Cahill.
Short of a 25-yard drive by Costa that flashed wide in stoppage time, the half-time whistle sounded without Chelsea having offered anything in attack.
Any hopes the visitors had of masterminding a turnaround vanished within four minutes of kick-off in the second half as Herrera doubled United's lead.
Zouma's tackle on the burrowing Young sent the ball rolling into Herrera's path and his drilled effort flicked off Zouma to fly past the grounded Begovic.
Chelsea manager Antonio Conte sent on Cesc Fabregas for Victor Moses, with Pedro moving to wing-back, but still the league leaders toiled, Pedro twice curling shots over De Gea's bar.
At the other end Rashford remained a constant menace, teeing up Lingard to bend a shot over and twice going close himself, ruffling the side-netting and testing Begovic at his near post.
When he departed in the 82nd minute, with Ibrahimovic taking his place, he was accorded a richly deserved standing ovation.