ROB Matthews was the unlikely hero with the bat as Blackwood inflicted a first defeat of the season on Newbridge in the Macey Shield.
Blackwood won the toss and batted and looked in all manner of trouble at 50/5 but Matthews and fellow bowler Neil Reid saved the day.
They helped the hosts reach 164/6 with Matthews ending unbeaten on 58 – claiming it was his first half-century since he was 12 – and Reid chipping in with 35.
Spinners Kyle Tudge and Rhodri Evans then did the damage with the ball as they bowled out Glamorgan and Monmouthshire Division One leaders Newbridge for 112.
Blackwood travel to Pontymister in the quarter-finals while Sudbrook will head to Panteg after chasing down Usk's 119-9 for the loss of five wickets in the 19th over.
The other remaining last eight tie sees Croesyceiliog host Newport while Chepstow have already secured a semi-final berth with victory against Monmouth.
Monmouth won the toss on a dark and miserable evening and with the significant threat of rain in the air decided on batting first.
In-form openers Josh Grey and Adam Roberts started very positively as they took the score from 42/0 off 6 overs to 113/1 off 15 overs.
Grey fell to Chepstow captain and former Monmouth opening bowler Jon Roberts (1/21 off three) for a well constructed 44.
From then on Roberts took the lead before he was bowled by Tudor Morgan-Huggett for a well paced 60, his innings consisting of eight 4s and one massive six.
Despite a run-a-ball 21 from Matt Jones, the Monmouth innings was pegged back well by the Chepstow attack and in particular Tudor Morgan-Huggett (1/17 off four).
They closed on a competitive looking 149/3 off their allotted 20 overs in what were tough conditions for both sides.
Monmouth made a solid start with the ball as they removed Glamorgan seconds and Wales opener Gareth Ansell for 5 as well as the dangerous Carwyn James for 13 to leave the game evenly poised at 47/2 off five overs.
The Chepstow innings however was transformed by Tyler Morgan-Huggett (64*) and D.Bennett (34) who took the score to 95/3 off 13 overs.
For Monmouth there were wickets for R.Kuttappu (2/47), captain Ben Turner (1/27) and King's Taunton schoolboy Henry Breeze (1/21) and some stand out fielding with catches for Adam Roberts and Dave Croudace (2), plus a sharp stumping for new signing Rhydian James.
However, the visitors had no answer to Morgan-Huggett's innings consisting of six 4s and two 6s, as his explosive hitting in the gloom propelled Chepstow to the required total in the 18th over.
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