Chris Nash scored his first hundred of the season as Sussex comfortably negotiated 59 overs at Arundel to reach 201-2 in their second innings and secure a draw with Surrey.
Surrey declared their first innings on 456-9 with a lead of 161, leaving Sussex having to bat through much of the final day for the third successive game to stave off defeat and maintain the only unbeaten record in Division One of the County Championship.
A first-wicket stand of 107 between Luke Wells and Chris Nash calmed any nerves and Nash went to a chanceless century, the 14th of his first-class career, just before Sussex declared and stumps were drawn.
Nash had only passed 50 once previously this season and had few alarms on a placid, slow surface, hitting 11 fours in his unbeaten 102 - which came off 177 balls.
Chris Tremlett had earlier scored his first half-century for three years as Surrey collected maximum bonus points for the first time this season.
Surrey needed 12 off the 110th over to secure a fifth batting point and Tremlett got the runs in two blows, lofting Steve Magoffin over deep midwicket for six before pulling his next ball over the rope at deep square leg.
Tremlett hit two more sixes and three boundaries as he reached a 29-ball 50 with Tim Linley, who hit Monty Panesar for six, giving him good support in a ninth wicket stand of 74 from just seven overs.
Tremlett eventually holed out for 54 while Panesar picked up the other two wickets to fall to finish with 5-142, his first five-wicket haul of the season.
De Bruyn ton
The England left-arm spinner had Zander de Bruyn taken at cover for 111, the South African having collected the single he needed to go to his first hundred of the season in the third over of the day. De Bruyn faced 202 balls and hit 11 fours and a six.
Sussex lost two wickets to left-arm spinner Ansari when they batted again. Wells took his aggregate against Surrey to 528 runs from his last six innings with a 91-ball 50, which included six fours, but four deliveries after getting there he was caught at slip by Gary Wilson who held a juggling catch at the second attempt.
Michael Yardy gave Ansari his second success when he pushed forward and was held at short leg with the score on 131 but by then Sussex were all but safe.
The result sees Sussex slip down to third in the Division One table, 10 points behind new leaders Yorkshire.