Nottingham Forest won 2-1 in the first leg at Bramall Lane on Saturday, thanks to goals from Jack Colback and Brennan Johnson.
In stoppage time, Sander Berge headed home a Morgan Gibbs-White corner, setting up an intriguing second leg.
One of these two teams will progress to play in the final at Wembley, with the winner earning a spot in the Premier League.
Team news
Forest received a boost during the first leg as Keinan Davis returned from a hamstring injury and played for nearly a quarter-of-an-hour.
Steve Cooper is virtually set to keep the same lineup as on Saturday, thus the Aston Villa loanee will likely remain on the bench.
Cooper did have injury concerns during the lead up to the first leg with Jack Colback and Ryan Yates missing the final game of the regular season, at Hull City, with groin and shoulder problems, respectively.
Yates looked to have struggled with his shoulder again during the first leg of this tie, but Cooper told the local media that the midfielder is okay.
“He’s in the same place as he was before the game. He was fit enough to play yesterday, and it certainly hasn’t got any worse,” said Cooper. “Try to declare him unfit!"
The Forest boss also stated that he has no fresh worries going into the second leg, indicating the Colback will be okay for the game.
“There isn’t anything new from the game," Cooper said. We were carrying a few injuries going into it, with the players who were unavailable yesterday. There’s no changes from yesterday. There’s the normal bumps and bruises, and a little bit of fatigue from the game.
“But that’s very normal for the day after a game. I’m fairly sure we’ll be as we were for the first leg.”
Forest are, however, still without Lewis Grabban and Max Lowe - albeit the latter would have been ineligible to feature against his parent club anyway.
Paul Heckingbottom will almost certainly make only minor changes to his Sheffield United starting eleven, although he may use the opportunity to bring in George Baldock at right wing-back.
The back three should remain unchanged, while experienced attacker Billy Sharp is unlikely to return from a calf injury.
Predicted lineups
Nottingham Forest: Samba; Worrall, Cook, McKenna; Spence, Yates, Garner, Colback; Zinckernagel; Johnson, Surridge
Sheffield United: Foderingham; Basham, Egan, Robinson; Baldock, Norwood, Fleck, Stevens; Berge; Gibbs-White, Ndiaye
Ones to watch
Johnson followed in the footsteps of his father David, who scored against Sheffield United in the play-off final first leg in 2003, by scoring in the first leg of this tie.
Forest would benefit from another goal from the 20-year-old, given he has yet to lose a game this season in which he has scored.
Berge scored after the 55th minute for Sheffield United for the first time in four games in the first leg.
The midfielder has made this tie much tighter than it could have been, and he will seek to make an even bigger impact in the second leg.
Previous meetings
The last time these two teams faced was on Saturday, when Nottingham Forest won 2-1.
Forest's victory was their first over Sheffield United at Bramall Lane since 2004, and they appear to have saved it for the most critical meeting.
In the 10th minute, Jack Colback pounced on a rebound from Philip Zinckernagel's close-range attempt.
Forest had six shots on target in the first half and will have been disappointed to just lead 1-0 at the break.
Sheffield United had eleven shots themselves in the second half, but Brennan Johnson was on hand to convert Joe Lolley's deflected cross in the 71st minute to give Forest a two-goal lead.
In the 91st minute, Sander Berge equalised from a Morgan Gibbs-White corner, setting up an exciting second leg.
How to watch
This match is on Tuesday, May 17, and kicks off at 19:45 BST. Coverage of the game will be available from 19:30 BST on Sky Sports Football.
What the managers have said
Nottingham Forest boss Steve Cooper spoke to the club's media ahead of this fixture, explaining how important his teams game plan is and the added advantage of playing at home.
He said: "Half-time gets mentioned quite a lot when we speak about play-offs, and at half-time in any normal game you would try and carry on. I think this is fairly similar.
"I think we've approached games this season in a certain manner which has served us well, so we won't come away from that. Preparations will stay the same.
"Taking a step aside from the game, it was a really good occasion on Saturday. It's great to be part of the play-offs and they're the games everyone tunes into.
"We're looking forward to the game. We love going back home and playing at The City Ground, and when the games have something on them like this one, we're even more excited.
"All we've got to do is be confident in our work, commit to the plan and believe in what we do. The spirit is really high, and the training ground has been excellent.
"We've had incredible support this season. There's been times where we've had to really work to get a result, but the fans have kept us going. Our support both home and away has been incredible, but it's our responsibility to try and get them feeling that way."
Ahead of this play-off semi-final second leg, Sheffield United boss Paul Heckingbottom spoke to his local media about his previous play-off experiences and how his side can be better than they were in the first leg.
He said: “In the three successive ones I’ve had, I’ve won the away leg every time so it can be done, we just have to be better out there.
“The slips can happen. We aren’t going to blame the defeat on two slips for the goals from Jack Colback and Brennan Johnson).
“We have to do what we know we can do better. We like to be the team on the front foot.
”Forest have got a lot of pace at the top end of the pitch so we have to be aware of our strengths more. We have to be better than them, that doesn’t change.
"The lads are in the dressing room saying to each other, 'we're still in this.' Look at how they celebrated their second goal, as if they thought they were through. It makes the first big incident at their place key now."