Sheffield United's David McGoldrick scored twice as they stunned Chelsea with a 3-0 win at Bramall Lane on Saturday, loosening the visitors' grip on a Premier League top-four place.
Chelsea's Champions League qualification hopes were badly weakened by the result as they could fall to fifth if Leicester City beat Bournemouth on Sunday and Manchester United overcome Southampton a day later.
A fourth straight home win in the league for Chris Wilder's side, however, boosted their hopes of a first-ever appearance in the Europa League as they moved up to sixth on 54 points.
Forward McGoldrick had not previously found the net in the Premier League since winning promotion with Chris Wilder's team but opened the scoring when Oli McBurnie's shot was pushed out by Chelsea keeper Kepa Arrizabalaga and the Ireland striker fired home.
McBurnie then doubled the lead in the 33rd minute with a powerful, angled header from an Enda Stevens cross.
The 32-year-old McGoldrick wrapped up the win in the 77th minute, slotting home after Chelsea defender Antonio Rudiger failed to deal with a low ball from Lys Mousset.
Chelsea manager Frank Lampard was scathing in his criticism of his team's performance.
"They were better than us, physically, in the mind and with the ball. So you lose the game," he said.
"They were stronger than us. You feel and hear every noise and all I could hear was Sheffield United voices. They are a good team. If you come here and perform below par this is what will happen.
"Disappointing. Mistakes. You analyse all season and review and move forward. We need to put that to bed quickly and fight for the top four. I learned a lot and I won't forget that," he said.
McGoldrick said his barren run had not perturbed him.
"I had told the lads ‘don't worry when it comes I will get a brace'," he said.
"Thirty-five games in and the league table doesn't lie. That was us at our best today."