Unbeaten Surrey were given an almighty scare as they just scraped home off the final ball to win the battle of South Group's top two against Somerset.
Despite Peter Siddle's three wickets in the last over at The Oval, Conor McKerr came in to smash his first ball for four as Surrey won by three wickets.
Leicestershire and Derbyshire were both victorious in North Group.
There were also wins in South Group for two of the main top-four contenders, Gloucestershire and Glamorgan.
Gloucestershire beat hapless T20 holders Kent at Canterbury, while Glamorgan also gave their hopes of qualifying a big boost as they hammered Middlesex by eight wickets in Cardiff.
Openers David Lloyd and Sam Northeast shared a Glamorgan T20 record stand of 150 as their side coasted home with 34 balls to spare.
Surrey's ninth win in 10 games officially ensured qualification for the last eight of this year's Blast, while Gloucestershire climb to third and Glamorgan are now within a point of fourth-placed Essex.
"I don't think I've ever felt so excited on a cricket pitch," said McKerr. "As it came off the bat and I knew it was going to the ropes I was screaming and running across the field."
Despite it being first against second at The Oval, the game was played out in front of a vastly reduced audience as a direct effect of the national rail strike.
Following 39 from opener Tom Banton earlier, Somerset had Lewis Goldsworthy and Siddle to thank for an unbroken ninth-wicket stand of 41 from 103-8 as they reached a more defendable 144-8.
It never looked likely to prove too stiff a target for the runaway South Group leaders - but it turned out very differently.
Only Will Jacks (66no) held firm as wickets fell steadily until Surrey were left needing 24 off 12 balls.
Jacks belted Jack Brooks for six off the first ball as Surrey took 15 off the penultimate over, needing only nine to win off the last over from Siddle.
But, after Jacks took a single off the first ball, Siddle then removed Chris Jordan, Nico Reifer and Gus Atkinson in the space of four balls.
Debutant Reifer had actually hit his first ball for four - and McKerr then did the same, crashing the winning hit through the covers past the despairing dive of Tom Abell.
Max Holden was the Middlesex mainstay with 58 as the visitors set a target of 171-7 against Glamorgan at Sophia Gardens.
But Lloyd and Northeast helped make mincemeat of it.
Skipper Lloyd hit two sixes and 10 fours in his 67 off 36 balls before perishing in the deep to the penultimate ball of the 12th over,
But Northeast carried on the carnage, finishing with three sixes and 12 fours in his 89 off 46 balls before departing with just five runs needed.
Ian Cockbain (64), Glenn Phillips (62) and Chris Dent (40) all helped Gloucestershire post a challenging 195-4 against Kent at Canterbury.
But the visitors then so nearly made a mess of it.
Holders Kent have so far proved a shadow of the side they were last season, having won just twice in this year's competition.
But, spurred on by Jack Leaning's unbeaten 37 off 18 balls, they took the game to the last ball as Gloucestershire so nearly lost their heads.
However, Ryan Higgins held his nerve at the death to limit Leaning to 13 from the final six balls as Kent lost by just five runs at 190-5.
Northamptonshire Steelbacks missed the chance to move top of the North Group at Derby as they lost to in-form Derbyshire Falcons, who leapfrogged them into second place with an impressive six-wicket win.
Saif Zaib provided the backbone of the Steelbacks' innings with his highest-ever T20 score, a blistering 92 off 58 balls, including four sixes, before being run out in the final over.
However, no other Northants batter was able to match his scoring power, Jimmy Neesham offering chief support with 28 off 20 deliveries as Derbyshire restricted their visitors to 186-7.
Derbyshire then enjoyed a match-winning partnership of 95 between Wayne Madsen and Shan Masood, who set the platform for Derbyshire's fifth straight win.
Madsen struck two sixes and 10 fours in his 73 off 37 balls, while Masood contributed 57 off 43 before Leus du Plooy guided the hosts to victory on 192-4 with an over to spare.
Leicestershire Foxes boosted their hopes of qualifying for the quarter-finals with a third straight win as they triumphed by 47 runs against East Midlands rivals Notts Outlaws at Trent Bridge.
Foxes slipped to 21-2 early on but Lewis Hill (39) and Arron Lilley (23) steadied the ship before Ben Mike hit three sixes in his 29 off 12 balls and Wiaan Mulder struck 27 as Leicestershire recovered to post 170.
Alex Hales led the Notts reply with 55 but the former England opener was unable to find a partner to share the load, with Samit Patel (12) and Ben Duckett (11) the only other batters to reach double figures.
When Hales was caught by Mulder off the bowling of Mike at 97-6 the game was up and Naveen-ul-Haq mopped up the tail with 4-24 as Notts were dismissed for 123 to slip to a fifth defeat in six games.
Leicestershire climb to fifth, level on points with fourth-placed Birmingham Bears but having played two games more, while Notts sit seventh with four matches to play, likely needing to win them all.
Northampton: Northamptonshire Steelbacks v Birmingham Bears (18:30)
Blackpool: Lancashire Lightning v Durham (17:30)
Worcester: Worcestershire Rapids v Yorkshire Vikings (17:30)
Trent Bridge: Notts Outlaws v Derbyshire Falcons (18:30)
Lord's: Middlesex v Essex Eagles (18:15)
Hove: Sussex Sharks v Surrey (19:00)
Taunton: Somerset v Hampshire Hawks (19:05)
Riverside: Durham v Notts Outlaws (18:30)
Northampton: Northamptonshire Steelbacks v Yorkshire Vikings (18:30)
Derby: Derbyshire Falcons v Lancashire Lightning (19:00)
Edgbaston: Birmingham Bears v Worcestershire Rapids (19:05)
Sophia Gardens: Glamorgan v Somerset (18:30)
Bristol: Gloucestershire v Hampshire Hawks (18:30)
Chelmsford: Essex Eagles v Surrey (19:00)
Canterbury: Kent Spitfires v Sussex Sharks (19:00)