Sunderland are within touching distance of Premier League safety after one of the most dramatic matches the Stadium of Light has hosted.
The atmosphere was electric as Sam Allardyce's side came from behind to seal a precious - and what could yet prove pivotal - three points.
Here's what we learned from the Chelsea win:
Sunderland need to finish the job:
When referee Mike Jones blew his whistle at full-time, the Stadium of Light roof almost came off.
More than 47,000 fans were inside the stadium to witness an historic, dramatic and pulsating comeback against Guus Hiddink's side.
But the great escape isn't in the bag just yet. Sunderland need two points from their remaining two games to guarantee survival, given their superior goal difference.
A victory at home to Everton on Wednesday night would relegate both Newcastle United and Norwich City. One last huge push is needed.
Wahbi Khazri has found his form just at the right time:
Big Sam had called for a moment of magic in the build-up and his prayers were answered in the 41st minute.
Khazri stepped up with a quite stunning volley to draw Sunderland level. The technique was superb - textbook - as the ball flew past Thibaut Courtois in the Chelsea goal.
Uncharacteristic defending let the Blues back in before the break but Khazri played a key role in making sure Sunderland won the match.
His work rate was immense, pressing and probing the Chelsea defence and causing problems from set pieces. He has regained his form just at the right time.
Victory down to Vito Mannone as much as anyone:
It could all have been so different had the Italian stopper not pulled off two great saves to keep Chelsea at bay in the second half.
They were leading 2-1 when he twice denied Diego Costa. Those saves proved key as Sunderland hit Chelsea with two goals in the space of three minutes.
Allardyce was quick to praise Mannone post-match and rightly so.
Victory was down to him as much as anyone.
Faith of Sunderland fans rewarded:
Sunderland supporters have had to endure more than their fair amount of dross in recent seasons. But they have stuck by their side and look to be rewarded once again.
The atmosphere inside the Stadium of Light was the best it has been in years. Both Allardyce and Hiddink heaped praise on the fans post-match.
They could have turned earlier in the season but they can see this Sunderland side - under the expert guidance of Allardyce - is giving its all to stay up.
Sunderland 's character will see them over the line:
Fight. Grit. Determination. Call it what you will, this Sunderland side has it in abundance.
They don't know when they are beaten. Defensive errors almost cost them against Chelsea but you couldn't fault the workrate, commitment or desire of the players.
They snatched a point late on at Stoke City before coming from behind against Chelsea - and that character looks set to carry them over the finishing line once again.
Fabio Borini loves playing against Chelsea:
The Italian forward has some record against his former club, scoring in each of his last three Premier League appearances against Chelsea.
Sunderland were clinical, scoring from three out of their four attempts on target.
Defoe, meanwhile, scored his 18th goal in all competitions - a staggering 13 more than any other Sunderland player.
Get him on the plane to Euro 2016. England could certainly do a lot worse.