DURHAM settled for steady progress in the first hour then accelerated as they added 116 runs without losing a wicket before rain arrived eight minutes before lunch.
Scott Borthwick was the dominant partner as he overtook Keaton Jennings, but the opener moved two runs ahead on 89 at the break.
On 221 for one, Durham lead by 353 and will keep an eye on the weather forecast as they ponder when to declare.
Despite left-armer Lewis Hatchett taking five wickets in the first innings, Sussex opened up with two similar bowlers in Steve Magoffin and Jon Lewis.
Resuming on 46, Jennings coped comfortably with Lewis’s in-swing and drove a wide half volley through extra cover to reach 50 off 117 balls.
But when the bowlers started to concentrate on moving the ball away from him he failed to connect with several drives and was lucky not to get an edge.
When he did get a thick edge off Magoffin when aiming for mid-on the ball flew over backward point for four.
Borthwick, who spent two hours over his overnight 22, was well settled by the time Hatchett came on and he drove the left-armer wide of mid-on to complete his 50 off 121 balls.
Sussex have not called on Luke Wright to bowl at all, but when their fourth seamer, Steffan Piolet, was introduced Borthwick hit him for three fours in an over. Two were driven gloriously through extra cover and the other pulled in front of mid-wicket. He passed Jennings in the 70s, but the opener moved ahead again with a reverse sweep for four off James Tredwell, who again posed no threat.