Glamorgan 183-7 lead Durham by 76 runs
Durham were held up by Glamorgan's nightwatchman, Lukas Carey, before scenting their first win of the season when they took three wickets in three overs.
Carey's exit for 49 was swiftly followed by Aneurin Donald and David Lloyd failing to score, and when Graham Wagg was yorked by Barry McCarthy just before lunch Glamorgan went into the break on 183 for seven, leading by 76.
Andrew Salter added onnly one run to his overnight 25 before becoming Matty Potts' third victim when he skied a pull to mid-wicket.
Following his 54 in Glamorgan's win against Worcestershire, Carey got after McCarthy, who had conceded 16 runs in 11 overs then went for 26 in two.
One fierce drive almost offered a return catch and in the Irishman's next over a slower ball was lifted over long-on for six.
But Carey got carried away and had his stumps splattered when he swung round an in-swinger from Chris Rushworth.
Donald was lbw to a shooter from McCarthy then Lloyd edged a drive for Ryan Pringle to hold a sharp chance at second slip.
Cameron Steel was unable to hang on to another sharp chance at cover with Colin Ingram on 29 and the South African reached lunch on 30.