The Blues had looked well set for the second win of the Frank Lampard era after going in 2-0 up at the break thanks to a Tammy Abraham brace.
However, Callum Robinson’s effort and a 90th-minute own goal from Kurt Zouma saw the Blades go back to South Yorkshire with a hard-earned point.
It hadn’t looked like being United’s day based on the first half, two mistakes at the back costing them. Firstly, goalkeeper Dean Henderson spilt Abraham’s tame header under minimal pressure and the Blues striker was quickest to react as he tapped in the opener on 19 minutes.
Despite holding their own for much of the half and going close when Robinson’s header flashed wide of the target, they went in at the break two behind. A long Jorginho ball wasn’t dealt with as both Jack O’Connell and John Egan went for it but only helped it on to Abraham, who buried the chance as he took his tally to the season to four.
The visitors were quick out of the blocks after half-time though as Robinson turned home Enda Stevens’ low cross a minute after the restart.
Abraham then missed a great chance to complete his hat-trick as Henderson made amends for his error with a fine save and Chelsea never got another chance to add to their tally.
Lys Mousset tested Kepa Arrizabalaga from range as Sheffield United dominated possession in the second half.
They got the goal their hard work deserved when Robinson whipped in a cross which Mousset and Zouma both went for but it was the leg of the Blues defender which deflected the ball into the net.
It is now one win from his first four matches for Lampard’s side, a shaky backline conceding nine, while for the Blades, they have lost just one of their first four matches since promotion.