The Blades doubled the lead through a Billy Sharp penalty after Andrew Hughes was controversially sent off for a foul on Rhian Brewster.

Alan Browne pulled a goal back from close range before Riis popped up at the far post to slot in Ched Evans' cross.

Despite dropping two points, Sheffield United move up to 11th and into the top half of the table on 33 points, ahead of 13th-placed Preston on goal difference.

Preston should have gone ahead after 12 minutes when Riis broke through and fed the recalled Scott Sinclair, but the winger blasted against the legs of Wes Foderingham.

The Blades broke immediately from Foderingham's save as Iliman Ndiaye forced Iversen to push his low shot past the far post, and it was not long before Bogle put the visitors ahead with an effort that beat the unsighted Preston keeper.

Billy Sharp scored his 121st Championship goal to equal the record since the second tier was rebranded in 2004 - his first came for Sheffield United 5,061 days ago

The visitors began to up the pressure, but Preston coped well until conceding the penalty late in the first half.

Bogle's pass was flicked to Brewster by Sharp and the former Liverpool forward broke through before going down under a tangle of legs with Hughes. Referee David Webb pointed to the spot and sent the defender off for denying a goalscoring opportunity.

Sharp's penalty saw the 35-year-old equal former Preston forward David Nugent's record of 121 goals in the Championship since the second tier was renamed in 2004. The striker got his first at this level in March 2008 in a 2-1 win over Coventry City.

Iversen made a good save from Ndiaye soon after the restart before Foderingham made an even better one two minutes later when he tipped over Browne's header.

Brewster should have had the game out of sight after missing three good chances in the opening 20 minutes of the second half - his third saw the club's record signing drag a shot inches wide of the far post from six yards out.

Daniel Johnson and Riis had chances in quick succession before Browne's powerful side-footed finish got Preston back into it after Johnson had cut the ball back.

And those earlier misses came back to haunt Paul Heckingbottom's side as Riis sealed the win before Sepp van den Berg did well to block a stoppage-time effort from United's Conor Hourihane.

Preston North End manager Ryan Lowe told BBC Radio Lancashire:

"I'm more pleased with the character and the resilience and the desire of the boys.

"It's a tough scoreline for you to go in 2-0 down, but it's also a tough scoreline for the opposition - it's happened to me on many occasions.

"I just asked the lads to give it everything they've got. We changed one or two bodies at half-time, changed a couple of things around and when we got that goal back we felt we'd get another couple of chances because they were always on the ascendency wanting to kill the game off.

"I always felt we'd get an opportunity to go down the other end, so it's that never-say-die attitude in terms of what we're trying to do and what we're trying to build.

"It is a penalty, it is a foul, but I was a bit disappointed in terms of the sending off because I felt the two of them had arms together, but what a fantastic reaction from the group.".

Sheffield United manager Paul Heckingbottom told BBC Radio Sheffield:

"We threw it away. We've undone so much good work at both ends of the pitch.

"It could have, would have, should have been three, four or five. There were some good chances from some good play and then two killer blows at the end.

"While there's one goal in it, it doesn't matter if there's 10, nine or eight men, they're going to take risks and take chances.

"We had more than enough experience on the pitch at the end to see it out, so it's a sucker punch, a tough one to take and undone in terms of performance exactly what we're talking about and what we want to see."