Tom Alsop equalled his highest first-class score of 150 to give Sussex the advantage on a hot day at The 1st Central County Ground.
The sun was high, and the pitch was low as Sussex piled on 407 for four against Leicestershire, the only side in the second division of the LV= Insurance County Championship still looking for their first win of the season.
For Leicestershire, it was a day of toil in the field. Chris Wright was their only faster bowler to impress, and they had to rely on their spinners, Callum Parkinson and Colin Ackerman, to give them a measure of control. There were too many no balls and too many four balls.
Haines and opening partner Ali Orr set the tone for the day when they put on 61 in the opening ten overs. But in the next over Wright surprised Haines when he brought one back into the left hander. The batsman was in obvious pain as he marched at high speed in the direction of third man before flinging off his gloves.
After a delay of some minutes, in which he received treatment, Haines resumed batting and looked in no obvious pain. But in the next over, at 70 without loss, he retired on 24 and was replaced by Alsop.
The injury might have upset the momentum of the Sussex innings but there was no sign of that as Orr opened the face of his bat to drive Wiaan Mulder to the cover boundary to raise the 100 in only the 17th over.
Two balls later Orr went down on one knee and hoisted Mulder over the short midwicket boundary on the pavilion side for his second six to reach his fifty off 58 deliveries. That Mulder over went for 19 runs.
Sussex went to lunch on 138 for no wicket after 26 overs and Leicestershire did not break through until the second over after the interval when Orr, sweeping, was lbw to Parkinson for an 85-ball 70, with nine fours and two sixes.
That brought Cheteshwar Pujara to the crease with a batting average of 120 for Sussex this season, including four centuries. And Pujara looked in such total control that it was a surprise when, on 46, with eight easy fours, he was lbw to off-spinner Ackerman.
Then Tom Clark went across his stumps and was lbw to the same bowler, this time bowling round the wicket and making the ball straighten.
At tea Sussex were 285 for three and Alsop, who was missed at slip off Parkinson when he had made 60, reached his second century for his new club shortly after the break, off 160 balls.
Leicestershire took the new ball at 339 for three and shortly before the close, with the score on 380, Alsop was eventually dismissed after another fine innings at Hove.