Ystrad are now eight points behind RGC in second place, but have two games in hands over their title rivals. However, they too have to go to Penallta before the season ends.
Three Andre Soroko penalties gave the home side a 9-0 lead and then added a try and conversion after the break to grab all his side's points. A last gasp try from Afon Bagshaw, converted by Mitch McGahan, ensured RGC at least got something out of the game.
It ended all square at Mountain Ash as the home side came from behind against visitors Gilfach Goch. Six penalties for the away XV gave them the lead going into the tense closing minutes, and while a last gasp try from Mountain Ash's Ross Davies levelled the scores, the missed conversion left the lowly placed side frustrated.
Elsewhere, a brace of tries from right wing Lee Bendon handed Rumney a comfortable 26-10 over Ynysybwl.
Blindside flanker James Bull helped Division 2 East leaders Bedlinog cruise to victory, scoring two of his team's five tries away at Rhymney. The 30-0 result gives the Foxes their third successive league win. It was a rout at Llantwit Fardre as Abercynon were sent packing 33-20, with try scorers for the home side including Marcus Caudle, Lewis James and Ben Lee.
In Division 3 East, winger Liam Marmion crossed over twice to help give Gwernyfed a bonus point 33-12 win over Talywain and there was home joy for bottom placed Croesyceiliog in a 18-16 win against Abergavenny.