Tom Alsop's 84 and an unbeaten 75 from Cheteshwar Pujara formed the backbone of Sussex's solid reply to Gloucestershire's 417 all out on an attritional day in Hove.
Sussex was 267 for five at stumps at the 1st Central County Ground, 150 behind, when Gloucestershire made a late charge with left-arm spinner Zafar Gohar taking two wickets in two balls with four overs remaining.
A slow pitch neutered the seamers, offering little movement and slow turn for the spinners, while the batsmen of both sides found it difficult to raise the scoring rate even when they were on the board.
Alsop became the fifth player to reach 50 without a century, failing to hold a short ball from six-footer Dom Goodman, who had earlier removed Tom Clark after the Sussex opener had made 53 from 124 balls.
Getting rid of Pujara, who looked ominously in touch, with the second new ball of the morning could be key to Gloucestershire's hopes of a decent first-innings lead.
The tranquillity of the pitch was underlined by the ease with which Gloucestershire's last two wickets added 109 in 22 overs early in the day, led by Gohar, who made his 11th first-class half-century.
He added 56 for the ninth wicket with Zaman Akhter and the only alarm came when a mix-up over a single down the leg-side left both stranded at mid-wicket, but Pujara's wild throw enabled them to make their ground. Gohar mixed cautious defence with some aggressive off-side shots as he plundered the short boundary for most of his seven fours.
After wicketless Ollie Robinson failed to break through, it was left-arm spinner James Coles who broke the stand with his fifth ball when he bowled Gohar for 60 with a full-length delivery. Offspinner Jack Carson took a clever overhead return catch to remove Ajeet Dale for his third wicket, but not before Gloucestershire had scored a fourth wicket.
Dale then bowled a superb opening spell down the slope and was rewarded when Tom Haines, who had scored centuries in the first two games of the season, was caught behind by Miles Hammond.
Dale was involved in a lively one-on-one with Clark after lunch as the left-hander struggled to find some form after scoring 12 in the season opener against Northants and a first-ball duck at Leicester last week.
Clark continued to sweep away the loose ball and appeared to have done the hard yards when he reached a 115-ball fifty that included a six off off-spinner Ollie Price. But when Goodman returned in the 37th over, Clark poked at a ball he could have ignored after putting on 108 with Alsop.
Alsop was more fluent, reaching a 100-ball fifty, and the bat finally began to take hold after tea when he and Pujara added 85 in 22 overs. It came as a surprise when Alsop mistimed a pull and Tom Price ran in to take a diving catch at long-leg to give the resilient Goodman his second wicket.
Dale returned to hit James Coles on the right hand and helmet in another full toss, but Coles added 52 with Pujara and had just driven left-arm spinner Zafar Gohar out of the ground when he was caught trying to clear the short square-leg boundary. Gohar bowled Carson on the back leg with the next ball but John Simpson survived to complete the hat-trick.