Glamorgan fought back in the evening session of day two of their LV= County Championship match at Hove after Sussex earned a first-innings lead of 358.
The hosts started the day with Steve Smith (68*) and James Coles (50*) with a lead of 98 after dismissing the Welsh county for 123 on day one.
The pair put on a partnership of 143 before James Harris trapped Smith LBW for 89. Coles went on to score 138 before Harris struck again.
Michael Neser dismissed Ollie Robinson for 13 to wrap up the Sussex innings - all out for 481.
Debutant Zain ul Hassan and Eddie Byrom put on an opening stand of 91 before Byrom was caught for 57. Marnus Labuschagne and ul Hassan remained unbeaten, steering Glamorgan to 118/1 at the close, trailing by 240.
"Obviously we're still behind in the game at the moment but I think we put in a solid session there to give ourselves a chance going into tomorrow," said Byrom.
"The goal will be to try and get as big a lead as possible and the pitch is looking quite placid at the moment so that's definitely possible, to look to get a lead.
"We've got a long batting line-up and then hopefully the lead is big enough to bowl them out.