Essex inflicted Sussex's first home Vitality Blast defeat of 2019 to frustrate them in their bid to qualify for the quarter-finals.
Will Beer and Reece Topley took two wickets apiece but a stand of 82 between Dan Lawrence and Ravi Bopara carried Essex to 168-5 - 10 or 15 more than they looked like getting at one stage.
In the Sussex reply, the early loss of Phil Salt, Luke Wright and Alex Carey was forgotten as David Wiese counter-attacked but the innings faded after he was out for a 36-ball 66. Mohammad Amir did much of the damage with 4-29 as the Eagles triumphed by nine runs.
The result leaves Sussex still needing at least one more win - possibly two - from their final three games to make sure of a top four place and a quarter final spot but they are still top - though their rivals are close on their heels.
Wright won the toss on a warm and sunny evening in front of another sellout 6,000 Hove crowd and asked Essex to bat first. Essex skipper Simon Harmer said he'd have batted first had he won it.
Sussex were without several big names - Jofra Archer was with England for the Ashes, and was doing very nicely too with six Australian wickets in the day; Tymal Mills had picked up a back injury and Rashid Khan had ended his latest Sussex spell.
But the other side of the coin was the man brought into the bowling attack to replace Khan - Australian seamer Jason Behrendorff, recruited earlier in the week for the final four group games and, if Sussex get through, the quarter-final.
Behrendorff was asked by Wright to bowl the first over, from the Sea End, and the first ball was dispatched for four through the offside by Tom Westley. The rest of the over was very tidy as he found his line and length and only one more Westley run came off it.
Topley's first delivery of the second over got the same treatment from Westley as Behrendorff's first ball, before Cameron Delport got off the mark with a nice pull around the corner. Delport the Eagles' leading run scorer going into the game, didn't last long, though, miscuing a pull off Topley in the fourth over to be caught by Phil Salt for nine.
Adam Wheater announced his arrival with a lovely cut for four off Chris Jordan, then Behrendorff was switched to the Cromwell Road to bowl down the slope. It suited him and towards the end of a tight over he ran out keeper Wheater for six with a smart bit of work.
The 50 came up for the Eagles in the seventh over with Lawrence joining Westley at the crease, but the introduction of spin in the form of Will Beer soon brought about the end of Westley, caught by that man Behrendorff for a useful 34 off 25 balls.
Now it was a rebuilding job for Essex by Lawrence and Ryan ten Doeschate against Beer and fellow spin twin Danny Briggs and they didn't find it easy getting the ball away - until Lawrence hit Briggs for two successive sixes, one straight down the ground, the other over Jordan at long on.
Soon, though, ten Doeschate became Beer's second victim, holing out to Salt for nine. Lawrence and Bopara found Sussex's bowlers and fielders in mean mood although one over-keen throw to Beer at the bowler's end resulted in an extra run which brought up their 100 but did little to spoil the veteran's figures of 2-19 from his four overs.
Lawrence decided it was time to attack and Wiese was on the receiving end, seeing each of the first two balls of his second spell go back over his head for six.Lawrence's 50 arrived off 29 balls, featuring two fours and four sixes as Behrendorff's final over proved rather wayward giving him final figures of 0-34.
Lawrence and Bopara's 50 partnership was completed in five overs and suddenly Sussex were not as in control of proceedings as they had been.
Topley and Jordan bowled well enough in the final few overs but both batsmen were fortunate to see a couple of edges fly away for fours through the unguarded slip area. The pair continued to swing the bat but Bopara was brilliantly caught at long-on by Jordan off Topley (2-32) for a 24-ball 45. Essex ended on 168 for 5.
The reply began badly for the Sharks when Salt was caught for six by skipper Harmer in the first over, giving Amir an early success. With a run rate of 8.5 an over to go for, Sussex could not afford too many slip-ups too soon.
Wright and Laurie Evans calmed nerves with an early four each before two overs were up but Wright soon succumbed, hooking an Amir delivery high into the night sky for Paul Walter to come in from deep mid-wicket and take the catch.
Things looked even more bleak when Carey suffered the same fate as the openers before him and saw Bopara pouch an awkward catch, the Australian departing for three.
Evans and No5 Wiese relieved some of the pressure with some fine hitting - the South African putting Walter away for four successive fours in a superb bit of counter-attacking in the fifth over. Despite the early setbacks the Sussex 50 came up a ball before the end of the six-over powerplay and Wiese's clean hitting took him past 2,000 T20 runs in his career.
Evans and Wiese continued to pick the right balls to hit, keeping the scoreboard ticking steadily with a mixture of singles and well-placed boundaries. At the halfway stage Sussex were 86-3, needing 83 to win.
The partnership was broken when it was worth 65 as Evans was caught by Harmer at mid-off off Walter, a good comeback by the bowler after that expensive first over. That brought in Harry Finch for his first Blast innings of 2019 but his unfortunate run of ducks continued when he was lbw to Harmer trying a reverse sweep.
Jordan was in at seven and had a watchful start before a pull for four brought up the Sharks 100. Soon another landmark arrived as Wiese reached 50 off 26 balls with his eighth four.
The pair were keeping the run rate around eight or nine an over with quick running between the wickets, though some sharp Essex fielding was restricting the number of boundaries.
Wiese led the charge with 17 coming from the 15th over, bowled by Beard, but he fell for lbw 66 to Harmer in the next - swinging the pendulum back in Essex's favour..
Jordan and Beer had plenty to do but successive fours by Jordan off Amir in the 17th lifted Sussex hopes once more - even if they were followed by two dot balls, one of which nearly had Jordan playing on.
Harmer's fine catch down low in the covers off the bowling of Adam Zampa accounted for Beer and when Jordan holed out to Bopara off Amir for 19 it was pretty much game over. Behrendorff was Amir's next victim as Sussex ran out of time and batters and Topley and Briggs were never likely to get anywhere need the 20-odd needed off the final over by Beard.
Next up for Sussex is a visit to Uxbridge on Saturday.