Dimitri Mascarenhas changed the course of a low-scoring Twenty20 Cup match at the Rose Bowl with four wickets as Hampshire scraped home by 15 runs.
Hampshire recorded the lowest score ever at the Rose Bowl in the competition but it still proved enough to see off the challenge of their south coast neighbours.
Hampshire made a disappointing 112 for five but Sussex, who must have fancies their changes of a comfortable fifth win, could muster only 97 for nine against some tight bowling by Mascarenhas and former Sussex pace bowler Billy Taylor.
The match was witnessed by a crowd of more than 11,000, the biggest of the Rose Bowl for the Twenty20 Cup.
But there was little excitement when Hampshire were reduced to 19 for three with Michael Carberry, Mitchell Stokes and Chris Benham all failing to reach double figures.
Dominic Thornely briefly revived Hampshire with a dominant innings of 37 which included a six off Michael Yardy and four fours.
Mascarenhas showed his versatility and value to Hampshire with an unbeaten 22 off 25 balls but Hampshire were only able to accelerate over the last three overs when they scored 34.
Sussex were always ahead of the run rates but lost wickets in their search for quick runs and both Chris Nash and top scorer Yardy, who made 37, were run out as Sussex attempted to hurry to victory.
With five overs remaining, Sussex were 71 for six and still requiring 38 but Mascarenhas halted the late charge by dismissing Carl Hopkinson, Luke Wright, Robin Martin-Jenkins and Yasir Arafat.
Sussex required 21 from the last over of the match from Chris Tremlett but managed only five to give Hampshire an unlikely win.
Hampshire owed a debt also to Taylor who rocked Sussex by getting rid of Chris Adams to a catch at long-on and Richard Montgomerie who was leg before after making a solid 20.
Victorious captain Shaun Udal said: "We felt even with a small total that we were in with a chance because batting was never easy at any stage in the match.
"Sussex were never very far ahead of us and we bowled and fielded tightly and gave them no chance to get away."