An all-round performance from debutant Dan Douthwaite brought Glamorgan back into the match on day three as Derbyshire slipped to 171-9 in their second innings and an overall lead of 203.
Douthwaite struck 39 with the bat and added useful runs for the seventh wicket with Tom Cullen (40) as Glamorgan were bowled out for 346, before he took 4/33 with the ball to leave the match on a knife edge heading in to the final day.
"It was a good feeling to get my uni work done (this week), but even better to go how I did today," said Douthwaite.
"It was nice to get a few with the bat, but you couldn't have told me I was going to take a four-for today after how I started, similar to the first innings.
"I was lucky enough to get another go, I came back stronger and I'm happy with how it went second time round in a longer spell than I'm used to, so I was knackered at the end of it.
"It was a good team effort, no-one was sure going out to field where the game was going but we've taken the game by the scruff of the neck.
"We spoke with Matt (Maynard, coach) at tea and the aim was to take five wickets in that session but things went a bit better than planned, so exciting to hopefully come back and play for a win on my debut."