But Llantwit hit back with a brace of tries through midfield duo Adam Harcourt and Eliot Davies and, although Jones added a penalty when Geraint George was yellow carded towards the end of the first period, they led by four points at the interval.
Llangennech had the wind at their backs and a one-man advantage after the break but it was Llantwit Fadre who extended their lead when playmaker Jason Pocock's second penalty was followed by a third try for full back Scott Burrows.
A second score from Rhys Davies and a conversion and penalty from Thomas cut the deficit to four points once more at 25-21 but Pocock guided his side into the semis with two more successful strikes as he took his personal tally to 16 points.
Glynneath avoided the same fate as their 1 West colleagues by seeing off the stiff challenge of high-flying 2 West outfit Maesteg Quins 19-13 to move a step closer to the Millennium Stadium. The same side came from behind to edge the Glamorgan Silver Ball Final against Saturday's opponents two years ago but this time around they were out in front earlier on as they led 19-10 at half time before holding on in a tight second half at Abernant Park.
Rhiwbina's stunning season continues apace as they raced to a 36-19 success against North Walians Nant Conwy 36-19 at Caedelyn Park. The Cardiff-based club sit top of 2 East with only one defeat all season and they were too good for the mid-table team from 1 North, with Nant Conwy making the journey home empty handed despite a gutsy display in the last eight.
Merthyr complete the semi-final final line up in the Plate competition after Amman United failed to raise a team for their battle with the Ironmen.
Llandudno will fly the fly the flag for North Wales in the semi-finals of the Swalec Bowl after they continued their impressive year with a 24-10 win over Ystradgynlais. Llandudno had lost just one of their 11 league games prior to the quarter-finals and they were good value for victory over an Ystradgynlais outfit who had themselves headed north in high spirits having been beaten only twice in their 5 South West campaign to date.
The competition's lowest ranked side, Hartridge HSOB, put up a brave fight at home to Old Illtydians before going down 30-15. Hartridge play their rugby in 6 South East and were always going to be up against it given that their visitors are mid-table in 4 East but defeat would have been a little harder to take given that it was their first of the entire season following 10 wins from 10 attempts in the league.
Clwb Rygbi Cymry Caerdydd were comfortable winners over Nantymoel as they backed up an earlier league victory over the same opposition with a 35-18 success at Llandaff Fields. Nantymoel will have an immediate chance for revenge on home soil in 4 South East this weekend but it is Clwb Rygbi who remain on track for a memorable day out at the Millennium.
Llantwit Major make up the final four in the Swalec Bowl after winning a hard-fought encounter with Pyle 30-21 as they put their disappointing league form behind them. The hosts had lost eight of their 12 ties in 4 South East but battled past a Pyle side who had made the short journey with a similar league record behind them.
Swalec Plate quarter-final results:
Glynneath 19 Maesteg Quins 13
Llangennech 21 Llantwit Fadre 31
Merthyr v Amman United - W/0 Amman United unable to raise a team
Rhiwbina 36 Nant Conwy 19
Swalec Bowl quarter-final results:
Clwb Rygbi Cymry Caerdydd 35 Nantymoel 18
Hartridge HSOB 15 Old Illtydians 30
Llandudno 24 Ystradgynlais 10
Llantwit Major 30 Pyle 21