With little dampness in the wicket early on, conditions were set fair for batting and Somerset's left-handed opening pair piled on the runs.
Trescothick looked in control until he attempted a flashing drive off Corey Collymore and edged behind.
That brought Hildreth to the crease, and he soon showed the form that won him a place on the England Lions tour during the winter.
Once Edwards went, Justin Langer added a fluent 44 before being caught behind, but Hildreth found further support from Zander de Bruyn, who was unbeaten on 37 at stumps.
Collymore was the pick of the Sussex bowlers, his 1-72 off 23 overs scant reward for a hard day's work in the field.