Boro were beaten at Bramall Lane as two goals in the final 15 minutes sealed victory for Sheffield United.
The home side were rewarded for a strong start with a goal just after the half-hour, but Boro responded well.
As half-time approached, we were level as Delano Burgzorg converted from the spot.
An even second half looked like it could go either way, but the hosts went back in front 15 minutes from the end and secured the points with a third goal in the closing stages.
Possession
49%Shots
10Shots On Target
4Corners
4Fouls
112
0
Cards
3
0
Kelechi Iheanacho was handed a start for his Boro debut, with Sam Iling-Junior also in the XI for the first time.
But it was the second-placed hosts who began on the front foot as they looked for early inroads.
Jesurun Rak-Sakyi drew an early save from Mark Travers, who also did well to palm a dangerous Gustavo Hamer corner to safety.
Hamer went closer still when he bent a shot narrowly wide from the edge of the area, while Travers held onto a Rak-Sakyi strike as the Blades continued to threaten.
They took the lead on 32 minutes when Rak-Sakyi stroked home, capitalising on a loose ball after Ryan Giles had dispossessed Callum O'Hare as he bore down on goal.
Boro responded well and battled their way back into it, with Hayden Hackney firing the first warning with a 20-yard effort which home goalkeeper Michael Cooper beat away.
The Teessiders had a golden chance to level when George Edmundson was pulled back by Jack Robinson and a penalty was awarded as half-time approached.
Burgzorg made no mistake from the spot as he rattled past Cooper to score for the third game in succession.
Sheffield Utd: Cooper, Harry Clarke, Ahmedhodzic, Robinson, Burrows (Davies 90'), Peck (Choudhury 90'), Vinicius Souza, Rak-Sakyi (Brereton 67'), O'Hare, Hamer (McCallum 82'), Cannon (Campbell 45')
Subs: McCallum, Holding, Brewster, Brereton, Davies, Campbell, Choudhury, Faxon, Brooks
Middlesbrough: Travers, Ayling, van den Berg, Edmundson, Giles, Morris (Forss 89'), Hackney, Iling-Junior (Whittaker 73'), Azaz (Barlaser 55'), Burgzorg, Iheanacho (Conway 73')
Subs: Barlaser, Fry, Whittaker, Forss, Conway, Glover, Neto Borges, Dede, McCabe
Boro were more than a match for their hosts for much of the second half, but were unable to carve out decent opportunities.
Rav van den Berg produced a pair of fine blocks to deny Sydie Peck and O'Hare, with the Blades gradually claiming the upper hand as the game progressed.
They produced what proved to be the winner with quarter-of-an-hour remaining as substitute Ben Brereton-Diaz latched onto a loose ball and lifted an effort over Travers.
Boro pushed for a second equaliser in the time which remained.
George Edmundson headed wide from a corner and a thunderous strike from Burgzorg was deflected over as we attempted to exert some pressure on the United goal.
The game became stretched and the hosts could have sealed the points when Tyrese Campbell was played in on goal, but Travers produced a superb one-on-one stop to deny him.
Boro went down the other end and Burgzorg tested Cooper from 20 yards, but the goalkeeper was equal to his effort.
The home side made the result safe with three minutes left as Anel Ahmedhodzic headed in from a corner.
