Glamorgan took a narrow lead over Worcestershire as the visitors fought back in the final session after looking down and out.
They start day two on 177-6, 68 ahead of Worcestershire’s paltry 109.
Michael Neser (4-40), James Harris (4-18) and Timm van der Gugten eliminated the Pears mid-afternoon.
But Glamorgan captain David Lloyd’s 48 proved the better scorer as Adam Finch (3-39) and Joe Leach (2-46) sparked a meltdown.
Billy Root (25 not out) and Neser (33 not out) then added 57 in the final hour to tip a dramatic day towards the home side, in only the second partnership to pass 40 on the day.
The decision to bowl at first seemed marginal, but Glamorgan’s in-form opening bowlers made the most of any wetness on the pitch.
Taylor Cornall, a late call from the side, dropped the front leg to Van der Gugten for six before a trio of clean slip catches accounted for the next three wickets, with Marnus Labuschagne’s low catch to dismiss the Pakistani star Azhar Ali particularly impressive and important.
Brett D’Oliveira played around a second-ball delivery from Neser, and when Glamorgan finally changed bowling, Adam Hose dragged James Harris’ first delivery to his stumps for a 15-ball duck.
Roderick and Waite managed to dig in until the interval at 66-6, but four balls after the break Neser produced the innings ball to clean Waite for 22.
Joe Leach (16) and Roderick took Worcestershire into high numbers while looking relatively serene, before Harris quickly finished the innings as he had Leach caught behind the side of the leg, forcing Roderick to get away sneak for the high score of 39, and trapped lbw Finch first ball.
Leach quickly got to work with the ball as Eddie Byrom took the lead, but Worcestershire gave Lloyd and Labuschagne early starts which gave strong slip chances.
Lloyd was soon driving confidently against loose pins and hit nine fours in his 48 in an 81 stand with Labuschagne before Leach came back to throw the captain on his second spell.
Remarkably, Worcestershire turned the game around after tea as Matthew Waite, whose first two overs cost 19, conceded just three runs in a six-over spell and grabbed the wicket from Labuschagne.
Finch struck to take out Sam Northeast, Kiran Carlson and Chris Cooke as Glamorgan cooled down while dropping five wickets for 32.
Neser’s straightforward approach and busy root managed to stop the batting slide at the end of a fascinating day.
Glamorgan assistant manager David Harrison told BBC Sport Wales::
“We didn’t really know what to do in the draw, but because of the dampness in the wicket and the rain we had, it was probably a good time for bowling. The way the guys have played as a collective was perfect, they held their own very well and caused problems in Worcestershire.
“David Lloyd gave us a good positive start, with Marnus in the form he is in. But the wicket is slow and Worcester played well after tea to tie us up.
“There’s still enough on the pitch even with the oldest ball, but this partnership between Michael Neser and Billy Root has put us in a good position.
“I don’t see the wicket getting much faster with two heavy rolls coming, in the past it has improved slightly.”
Worcestershire assistant coach Kadeer Ali told BBC Hereford & Worcester:
“It was a very weird day, we were disappointed with the stick even though Glamorgan played very well, Gareth Roderick hit very well but we were a bit soft with our dismissals and we didn’t start well with the ball.
“Then at the end of the day we showed a bit of spirit and character, the boys played well and showed a bit of a fight.
“Before tea our pitches were really full and we got pushed to the ground and hit mid-wicket. So we had a good tea chat and hit more than one pitch so it was chalk and cheese.
“Across all three departments, Glamorgan were better than us, we lost a few big chances but we’re still in the game.”