Sussex are out of the Twenty20 Cup despite belatedly finding some form with the bat at Hove last night.

Matt Prior's 73 was the highest score by a Sharks batsman in competition history and their total of 173-6 their highest since they made 180-6 in the inaugural season - 17 games ago.

But Prior's effort was surpassed by a superb innings from Ronnie Irani who hit the final ball of Essex's run chase for his 13th boundary to bring up his century off 61 balls and see his side to a four-wicket win.

Irani's performance guaranteed Essex top spot in the south group and a home tie in the quarter-finals but Sussex have still not reached the knockout stages in the four years of Twenty20.

A week ago they were on course to end that sequence but three successive defeats - including two at Hove - meant they finished fourth.

Not that the 5,000 or so spectators at a sunlit County Ground seemed to mind too much and the last over was worth the admission money on its own.

Essex looked home and hosed when the penultimate over from James Kirtley went for 14 to get the target down to eight off six balls.

Irani sent the first delivery of the last over from Yasir Arafat sailing over the mid-wicket rope for his second six to leave Essex needing just two runs off five.

He scrambled a single off the next ball to level the scores but Arafat kept things interesting by removing Ryan Ten Doeschate and James Foster with successive deliveries.

But Irani had managed to scramble through to get back on strike and swung the final ball through the leg side to reach his first Twenty20 century.

Mark Pettini and Darren Gough helped Irani lay the platform for victory by helping him add 44 and 37 for the first two wickets but Sussex deserve credit for the way they fought back to take it to the wire.

Robin Martin-Jenkins took two wickets and Mike Yardy did a great job in halting Essex's charge. His four overs of left-arm spin cost just 19 and brought him the wicket of Will Jefferson.

But this was Irani's night. He chanced his arm towards the end but always seemed to find a boundary when one was required. Essex, in front of a partisan Chelmsford crowd, will take some stopping in the quarter-finals.

What has cost the Sharks a chance of progressing is their lack of runs and the absence of Murray Goodwin for three of their games left them short of experience at the top of the innings.

They had averaged a modest 125 in seven games before last night but at least Prior was able to improve that statistic.

This competition seems tailor-made for a batsman who loves to improvise and hit the ball a long way so it is a surprise that this was only his fourth half-century in 23 Twenty20 appearances.

Clearly mindful of the start they had made at Chelmsford a fortnight ago when they were 18-4 and out of contention after just four overs, Prior and Goodwin proceeded carefully against new ball pair Andy Bichel and Gough and were a modest 38-0 when the fielding restrictions were lifted after six overs.

The same slow pitch which had been used for the previous two Twenty20 games was supposed to help bowlers who took the pace off the ball but the five overs from Essex's spinners cost 60 as Sussex suddenly took off and the crowd began to enjoy themselves.

Goodwin sliced a drive to mid-off in the seventh over but Prior and Adams then added 73 in the next seven.

Their 50 partnership came off 33 balls and for a while they matched each other blow for blow. Prior hit eight fours and a six off Grant Flower in a 33-ball 50 while Adams cleared the mid-wicket rope and hit three boundaries on his way to 36 off 25 balls before playing across a straight one in Jahid Ahmed's first over.

Prior looked on course for the first hundred by a Sussex batsman in Twenty20 but took one risk too many and lost his off stump trying to sweep a yorker from Gough. He hit 11 off the 44 balls he faced to the boundary.

Irani may be a liability in the field these days but he would probably find a place in any all-star Twenty20 side because when that heavy bat of his connects the ball tends to go a long way. He set his stall out to anchor the innings and did exactly that.