Hampshire 260 for 7 (Prest 76, Snater 3-46) beat Essex 257 for 8 (Snater 64) by three wickets.
Hampshire once again showed their insouciance as they chased down 257 to set up a home knockout fixture while knocking Essex Eagles out of the Royal London Cup.
Hampshire had found themselves behind the rate despite Tom Prest's 76 and required 78 from the last 10 overs, on a pitch difficult to score on.
But Toby Albert - who had match-winning 84 and 65 not outs against Derbyshire and Lancashire - continued his clutch form with 34 in 26 balls before Felix Organ, Ian Holland and Scott Currie took their side to a three-wicket victory.
Hampshire, who were already through despite losing for the first time against Glamorgan last time, will play a home semi-final if they beat Yorkshire at Scarborough on Tuesday, or a home play-off if they don't and Lancashire defeat Kent.
Hampshire found timing the ball a chore almost from the off of their chase, as Nick Gubbins was brilliantly caught at slip by Khushi in the fourth over before Ben Brown was bowled.
Aneurin Donald added some impetus with a slog-swept and on-driven sixes and Prest looked at his most fluent during their 54-run partnership, the required run rate had gradually inclined though.
Donald was run out attempting a push and run and Fletcha Middleton's attempt to wrestle back the rate saw him sky straight up.
Prest reached his third fifty of the competition in 73 balls before he hoisted to long off, having twice been dropped during his innings of 76. Victory seemed a distance off.
But Albert's flair in a vital 60-run partnership with Organ took Hampshire to 31 needed from 28. That was dragged down to 12 needed when Organ was caught at midwicket for 30.
That dozen was knocked off by Ian Holland and Scott Currie with ten balls to spare, and took Hampshire back to the top of Group B. Essex had chosen to bat and began with unbridled run-scoring with Feroze Khushi and Robin Das rushing to 44 inside six overs but a bowling change to Jack Campbell reversed the momentum.
The fast left-hander struck with his fifth delivery when he found extra bounce to have Khushi caught behind before getting Tom Westley to loop to midwicket - both wickets sandwiched by Das flicking Ian Holland to deep square to leave Essex 48 for three.
Grant Roelofson, the Eagles' leading run-getter in the competition, lived a charmed life as he was dropped on five, eight and 29 before Toby Albert finally clung on under a steepler at deep square.
Nick Browne and Luc Benkenstein had also departed by this point, with Essex 112 for six, with Aron Nijjar following when leg before to Nick Gubbins' leg spin.
But Shane Snater and Richards recovered things and took their side towards a par score. Netherlands international Snater caught the eye with a huge slog sweep but it was his intelligent gap finding that accumulated runs quickly.
His maiden format fifty came in 51 balls during a 96-run stand with Richards - who only scored one boundary in his unbeaten 31. Snater was eventually caught at cover for 64 but Ben Allison thwacked two sixes to take Essex to 257 for eight.
