After their 14-0 win away at Dowlais, Morriston now find themselves in the semi-finals and with a great chance of going all the way. Sean Rees and Chris Curran got the tries for the away side against their Division Four South West opponents.
A rousing second-half comeback sent Lampeter into the final four, as they beat Usk 28-24.
Converted tries from Emlyn Jones, Andy Farrel and Pearce had given Usk a 24-15 half-time lead, with Time Evans, Jason Thomas and Aled Morgan on the trysheet for Lampeter.
The Division Three West side came out all guns blazing and finally broke down their opponents with a superb solo try from Gareth Griffiths.
Usk battled hard to hold on but lost the lead for the first time with just six minutes remaining when Gary Davies forced his way over the line. A last minute penalty from Hugh Thomas cemented Lampeter's place in the semi-final draw.
Pontyclun will be the lowest-ranked side in the semi-finals after their 17-5 win away at Gowerton and Glyncorrwg complete the final four, they survived a late comeback to beat Rhiwbina 19-15 at the Recreation Ground.