Notts Outlaws, Surrey, Lancashire and Gloucestershire won their respective T20 Blast quarter-finals to reach Finals Day at Edgbaston on Saturday.
Notts against Leicestershire finished as a tie with both teams making 139-7 at Trent Bridge, but Notts' better Powerplay score edged them through.
Elsewhere Surrey beat Kent by 56 runs for their eighth win in a row.
Lancashire beat Sussex by 45 runs at Hove while Gloucestershire beat Northamptonshire by seven wickets.
The two semi-finals and final are due to be played on Saturday behind closed doors, although the forecast in Birmingham is poor and Sunday's reserve day may have to be used.
Surrey will face Gloucestershire in the first semi-final while Notts Outlaws play Lancashire Lightning in the other tie.
Last-over drama at Trent Bridge
While low-scoring on paper, Thursday's last quarter-final between Notts Outlaws and Leicestershire Foxes served up the closest finish.
In scenes evoking the end of the 2019 World Cup final, hosts Notts were declared the winners despite both sides finishing with exactly the same score.
But there was to be no Super Over or boundary count-back this time. The equation which put Notts into the semi-finals was the fact they had bettered Leicestershire's 22-2 in their first six overs by making 42-1 themselves.
A game of such fine margins was firstly due to Leicestershire bowling and fielding resolutely to defend 139, before two unfortunate misfields from Arron Lilley and Dieter Klein in the last seven balls.
Notts had slumped from the seemingly comfortable position of 58-1 to 99-6 and when Steven Mullaney fell to Gavin Griffifths (3-21) at 123-7, Notts still needed 17 to win off seven balls.
But Lilley, who had just taken a fine catch in the deep to dismiss Mullaney, then let a ball slip between his legs on the boundary for four to make it 13 needed off the last over.
Samit Patel (28 not out) struck a six over cover off Will Davis to bring it down to four needed off two balls. But the experienced all-rounder lost the strike for the last ball, meaning Imad Wasim needed two runs to tie the scores.
At first, it looked like he had not found a gap, but Klein was unable to gather the ball cleanly running in from long-on and Imad and Patel scampered back in time to ensure Notts reached the last four by "the barest of margins" as Ian Smith once famously said.
It was a cruel way for Leicestershire to miss out after they had earlier scrapped their way to 139-7 thanks to Harry Swindells' 58 off 43 balls and Colin Ackermann making 43 off 33 deliveries.
Ackermann also took 3-27 with the ball, but his and his team's efforts were ultimately in vain.
Jacks spins Surrey to Finals Day
England opener Jason Roy struggled for runs during the 50-over international series against Ireland and Australia, registering a highest score of 24 in six innings, but his knock of 72 in Surrey's final T20 group game against Kent signalled a return to form for the 30-year-old.
Against the same opposition, Roy added 56 in a century partnership with former South Africa batsman Hashim Amla (73 not out) to help his side to 169-2 at The Oval.
Off-spinner Will Jacks then accounted for Kent's main attacking threats at the top of the order, dismissing England trio Zak Crawley, Joe Denly and Sam Billings, as well as Daniel Bell-Drummond - the leading run-scorer in the group phase.
Surrey will go to Edgbaston as the in-form team, having now won eight consecutive T20 games.
Bowlers were on top at Hove, where Sussex restricted Lancashire to 140-8 in an innings disrupted by a rain delay.
But Sussex were then undone by the visitors' spinners during their run chase, with Liam Livingstone (4-23), Matt Parkinson (3-9) and Tom Hartley (1-18) sharing eight wickets between them as the Sharks were skittled for 95.
Meanwhile, Gloucestershire ended their last-eight hoodoo in the T20 Blast after losing at the quarter-final stage in three of the previous four years.
Victory over Northants rarely looked in doubt after a superb effort bowling first in Bristol.
Benny Howell (3-16), Ryan Higgins (3-24), David Payne (2-14) and Graeme van Buuren (2-17) helped bowl out Northants for 113 in the final over and the home side reached their target with more than eight overs to spare.