An explosive half-century from Jason Roy helped Surrey to a three-wicket victory over Worcestershire Rapids in their NatWest T20 Blast quarter-final at The Oval.
Roy's stunning 52 from 23 balls, including three sixes and eight fours, took his run tally to 619 from 14 innings in this summer's Twenty20 competition at an average of 47.61 and helped Surrey overhaul Worcestershire's modest 141-9 with 3.3 overs to spare.
There was a little bit of a wobble after Roy's exit, with Worcestershire fighting hard in the field to take seven Surrey wickets on a slowish pitch, but Robin Peterson finished matters in the 17th over by swinging Shaaiq Choudhry's left-arm spin for six and then driving the next ball through mid off for four.
After the six-over powerplay at the start of their innings, Surrey had rushed to 68-2 thanks to Roy's fireworks.
When Roy was out, mis-hitting Jack Shantry to mid-on in the sixth over, it was left to Pietersen and the rest of Surrey's international-packed middle order to complete the job.
Pietersen hit 29 from 27 balls, with five fours, but he then pulled Joe Leach to mid-on at the start of the 12th over and Surrey made rather heavy weather of the remaining chase.
Surrey were 104-4 when Pietersen departed, with Gary Wilson already back in the dugout and, when Rory Burns was brilliantly run out for two by Brett D'Oliveira's direct hit from cover, the scoreboard suddenly showed 106-5 and the Rapids sensed they still had half a chance.
Worcestershire captain Daryl Mitchell brought his field up knowing wickets were their only hopes of victory, a ploy which accounted for Azhar Mahmood and Zafar Ansari in quick succession but there were no further alarms for the hosts as Peterson led them home in the company of Gareth Batty.
Having been put into bat, Worcestershire suffered from losing wickets at regular intervals, with Jade Dernbach, Matthew Dunn and Peterson claiming two wickets apiece.
A 36-ball 20 from Richard Oliver and a 16-ball 20 from Leach were the only real contributions in a stop-start innings than never really gathered momentum for the Rapids.
Jordan Clark fired Lancashire into Finals Day with four wickets in eight balls without conceding a run as the Lightning came from a long way behind to beat Glamorgan by one run in a thrilling quarter-final at Emirates Old Trafford.
Thanks in large part to Clark, Lancashire were able to defend a modest total of 137-8 - built around Usman Khawaja's 67 off 54 balls - to progress in a match delayed for 24 hours due to rain.
All-rounder Clark, 23, was introduced into the attack for the 14th over of the Glamorgan chase with the visitors seemingly cruising at 88-2 and with Jacques Rudolph unbeaten on 33.
However, Clark's seamers accounted for the wickets of Murray Goodwin, Chris Cooke, Stewart Walters and David Lloyd as the Welshmen slipped to 100-6 after 16 overs and could not recover.
The latter three dismissals were part of a triple-wicket maiden for the bowler who had only taken four wickets in 10 previous Twenty20 spells throughout his four season career.
Earlier, Lancashire's Australia batsman Khawaja hit a career-best 67 with six fours and a six.
Lancashire never really got going in their innings, aside from a third-wicket stand of 40 in 29 balls between Khawaja and Karl Brown to advance the score from 19 for two early in the fourth over.
They lost wickets regularly from the eighth over onwards and stumbled as the Glamorgan attack expertly took the pace off the ball.
The Birmingham Bears ensured they will have home advantage later this month as unbeaten half-centuries from captain Varun Chopra and Rikki Clarke proved too much for Essex to overcome, despite the best efforts of skipper Ryan ten Doeschate.
Chopra hammered six fours and four sixes in his 86 off 54 balls, while Clarke was even more dynamic with eight boundaries and two maximums in his 70 off 37 deliveries to guide the Bears to a formidable 197-2.
The hosts were undermined by a steady fall of wickets and although ten Doeschate (61no off 37 balls) ensured they did not capitulate, Essex fell well short and finished on 178-5.
The winners of Nottinghamshire's clash against Hampshire on Sunday will complete the line-up for Finals Day, which will be held on August 23 at Edgbaston.