Southampton continued their march up the Championship table as they came from behind to beat fellow play-off hopefuls Derby 2-1 at St Mary's.
Second-half goals from Leo Scienza and Taylor Harwood-Bellis cancelled out Carlton Morris' earlier strike to extend the Saints' unbeaten run to 17 games.
It was a fast start from the hosts as Scienza got in down the left and pulled the ball across for Tom Fellows, whose heavy first touch cost him the chance to shoot.
Derby were forced into a ninth minute change when Bobby Clark limped off to be replaced by Oscar Fraulo.
The Rams started the day just three points behind Southampton and showed plenty of intent as they bid to secure a crucial victory.
A smart exchange of passes between Morris and Max Johnston saw the latter break clear down the left channel before he shot wastefully into the side netting.
Five minutes later, a sweeping Saints attack involving Casper Jander and Ross Stewart saw a chance fall Finn Azaz's way but he shot straight at Richard O'Donnell.
Derby were without United States striker Patrick Agyemang, who has been ruled out for the next nine months with an Achilles injury, denying him the chance to represent his country in a home World Cup.
Morris was tasked with leading the line for John Eustace's side in his absence and scored an unlikely opening goal when Joe Ward's corner deflected off Jander and dropped to him at the far post.
Saints sought a quick reply and went close as Harwood-Bellis nodded Ryan Manning's free-kick wide of the post as the wind swirled around St Mary's.
Again it was Southampton making all the running at the start of the second half and Scienza laid on a chance for James Bree, who sliced his shot well wide of the post.
Another chance soon came and went when Fellows headed Manning's cross over the bar from close range.
Derby were seeing little of the ball in the Southampton half but there was a rare test for Daniel
Peretz when he saved Sammie Szmodics' 20-yard shot low to his left.
Moments later, Saints got the goal they deserved when Scienza brilliantly finished off a flowing move by curling into the far corner after Jander and Azaz had created the opening.
St Mary's erupted once more when Saints went ahead as Harwood-Bellis this time connected perfectly with another Manning free-kick to head beyond O'Donnell.
Eustace quickly rang the changes as he looked to find a way back into the game and Harwood-Bellis reacted superbly to block Morris' effort from the edge of the six-yard box.
However, the Saints remained resolute and are now unbeaten since January 17.
Sheffield United's Chris Wilder:
"We got off to the worst possible start. Hull are in the position they are for a reason; they're a team that like to counter and suck you in and they've got talented players at the top of the pitch.
"It could have been 4-4 at half time, they had some big chances but we had some really good opportunities as well. They've not had to work as hard for their opportunities so we tightened it up in the second half.
"We managed to get hold of the ball and we drove the game forward. We kept working and kept pushing and luckily we managed to find ourselves a winner at the end.
"I know people will talk about the red card and the penalty but we've had so many decisions go against us this season. We're never talking about games anymore; we're always talking about the referees because they're missing so many decisions. We'll take this one bit of luck with open arms.
"It's been a tough little period for us. This is an expectant football club and we've not won in six games. We haven't got the points we should have done, not only over the last six games but all season.
"The play-offs might have gone for us but the message is as strong as ever. There's always something to play for and we're not going to wave a white flag. I want to win every game for the rest of the season."
Hull's Sergij Jakirovic:
"It is unbelievable for us because we were the better team today. We had so many chances and the game has been changed completely by the referee.
"John (Lundstram) gets a second yellow card because the referee gets in the way. What is he supposed to do in this situation?
"I don't know what happened with the penalty but I saw six players on the ground. If a penalty was always given for this there would be 10 penalties every game.
"Referees don't want to speak with you about decisions, they tell you to go away. This is a problem for all head coaches in the Championship.
"We could have been more clinical and finished the game earlier. We lost the game in the end and it is not easy to manage your emotions when you know you're the better team and you leave with nothing.
"If we play like this for the rest of the season, I have no worries. We have four games to go and in the last two games we have been very good, we just have to keep continue playing like this and the points will come."
