Raiders had picked up two victories from the first two games at Roundstone Lane with wins over Old Elthamians and Cornish All Blacks.
However, a fine away-day display from Southend Saxons saw them become the first team to win at Worthing this campaign.
The defeat means Raiders slipped to a second successive loss, having secured three straight league wins in previous weeks.
It all started so well for the Raiders and they registered the first points of the afternoon on six minutes.
Iain Greenwood intercepted a misplaced Saxons pass before passing to Samuel Aspland-Robinson, who ran 30-yards before going under the posts.
Sean O’Hagan kicked the conversion wide, meaning Worthing only had a five-point lead.
Raiders continued to dominate for the next ten minutes but the game changed when Scott Barlow received a yellow card for dangerous play
This gave Southend a man advantage for ten minutes and they made the most of it.
Just three minutes after Barlow was dismissed, Aston Bevan-Royston went over, while Bradley Burr converted, to put his side 7-5 in front.
Raiders responded well to going behind, though, and were quickly back ahead thanks to an O’Hagan penalty.
Southend made the most of that man advantage a minute before Barlow’s return.
Harry Hudson broke clear of the Worthing defence and found Dan Legge who went over.
Burr made it two conversions from two, to give the travelling side a 14-8 lead.
It got better for Southend right on the stroke of half-time as Burr kicked over a penalty to ensure they reached the interval 17-8 to the good.
Worthing knew they needed to pick up some early points in the second period for any hopes of a comeback.
And it looked as though Raiders would claw themselves back into it, with O’Hagan converting a penalty on 57 minutes.
This left them trailing by just six points with just over 20 minutes left to play.
However, their hopes were dealt a huge blow when they were reduced to 14 men for a second time.
This time around it was Liam Perkins who received the yellow card for a deliberate knock-on.
Worthing were now left with it all to do with 18 minutes left to play.
Burr made no mistake with the resulting penalty and ensure his side opened up a nine-point lead.
Once Perkins returned from the sin bin, Worthing piled men forward.
But resolute defending and some poor passing from the home side cost them dearly.
A third yellow card of the contest was brandished with two minutes left.
It was not a third for Raiders but this time Legge of Southend was given his marching orders for dangerous play.
O’Hagan had a chance to reduce the arrears late on with a penalty, though, he kicked wide and it ended 20-11 to Southend.
Worthing will be hoping to bounce back from this defeat when they travel to Bury St.Edmunds in the league on Saturday.
RAIDERS: Storer; Watson-Brooks, Sargent, Barlow, Heward, White, Hewick, Perkins; Leeming, O’Hagan, Aspland-Robinson, Greenwood, Loosmore, Wilcockson, Farnes. Subs: Whittall, Lake, Margetson, Govett, Miles.
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