Lewis Grabban scored the only goal of the game as Sheffield United were beaten 1-0 at Nottingham Forest's City Ground this afternoon.
The £6m striker's header in the second half proved the difference between Forest and the Blades, who had gone into the game top of the table two points clear of their nearest rivals.
But it was Forest, who started the day six points and six places behind the Blades in the table, who began the brightest with Matty Cash, on the right wing, a threat throughout.
Paul Coutts, returning to the Blades starting line-up as one of three changes alongside John Egan and debutant Martin Cranie, saw a well-struck shot blocled by a Forest defender after Billy Sharp's lay-off, and Joe Lolley hit a thunderous volley at the other end that Dean Henderson in the Blades goal was relieved to see sail over his crossbar.
The United goalkeeper easily claimed Jack Robinson's effort just before half-time, but the suckerpunch came when Grabban headed the hosts in front.
Kieron Freeman conceded a needless corner with a poor defensive header and didn't do enough to stop the resulting cross from Joao Carvalho, which allowed Grabban to get the deftest touch and beat Henderson with his header.
United almost rescued a point in injury time when a cross was knocked down in the Forest area, but neither Sharp nor substitute Ben Woodburn could convert.
Forest: Pantilimon, Darikwa, Dawson, x, Robinson, Cash (Dias, 83), Lolley, Guedioura, Yacob, Carvalho, Grabban (Ansarifard, 90+). Not used: Steele, Osborn, Janko, Tachtsidis, Hefele.
Blades: Henderson, Freeman (Baldock, 80), Cranie (Woodburn, 80), Egan, O'Connell, Stevens, Coutts (McGoldrick, 55), Norwood, Fleck, Sharp, Clarke. Not used: Moore, Stearman, Johnson, Washington.