Ian Holland opened the attack with Helm and frequently Hughes jumped down the pitch to attack the bowling of the Leicestershire captain.
While the ball was still hard there were problems for the Sussex batters. Jack Leaning struggled in particular in the opening hour, and he might have been fortunate to survive a plausible lbw appeal from Green.
At lunch Sussex were 125-2, with Hughes 77 not out and Leaning unbeaten on 28. Hughes reached his century – out of 154 – when he square-drove Holland to the point boundary.
When Ben Cox fluffed a stumping chance off the bowling of Ajaz Patel, with Leaning on 44, the visiting side must have wondered where the next wicket would come from.
But Sussex came to their aid. Leaning had just reached a patient, obdurate fifty, off 131 deliveries with five fours, when he gave it away, going down the wicket to drive a simple catch to mid-on and leave his side 196-3. His innings – and his demise – replicated Holland’s effort the previous day.
There was more where that came from. Hughes had been batting at his best and looked set for a massive score. But when he had made 136 he drove a wide half-volley to cover, with the score 224-4.
The Australian left-hander looked crestfallen. But he had played a gorgeous innings, batting for 227 minutes and hitting 20 fours.
The out-of-form James Coles, dropped a place to number five in he hope of finding his touch, looked in the mood when he expertly guided Tom Scriven to third man for four.
But he had made only 19 when he attempted a lavish off-side stroke off the back foot and edged to Cox, who took a splendid catch.
At 245-5, Sussex appeared to be throwing it away, losing three careless wickets in the space of 49 runs. But some of the best batting was still to come. John Simpson, batting with power and easy timing, put on a sprightly 88 in 20 overs with Charlie Tear.
Helm ended the fun when he took the new ball, at 333-5, and had Tear lbw on the back foot for an eye-catching, 67-ball 44.
Fynn Hudson-Prentice and Jack Carson fell cheaply but Simpson was still there at the close, unbeaten on 89.
