Corus were the only side to beat Tonmawr in Division Three last season, but they fell foul of Owain Higgins's deadly accurate boot as he landed five penalties and a conversion in the home side's 22-12 triumph.
Meanwhile, Tonmawr outscored their hosts by three tries to two - centre Will Price, wing Darren Griffiths and No.8 Rob Hill crossing the Athletic line - and were able to overcome a yellow card to claim a great win.
The new boys in Division One East, Rumney and Llantrisant, were both beaten. Rumney fell 20-13 at home to a Bargoed side that lost two players to the sin bin, while Llantrisant conceded five tries in their 32-12 defeat at Caerphilly.
Centre Gareth Miles and wing Steffan Sankala both crossed the Llantrisant line twice and there was a debut try for the Cheesemen from Rob Phillips, brother of Wales's Rugby World Cup scrum half Mike.
Reigning Division One West champions Bonymaen were held up on the M4 because of an accident on their way to Narberth, but there was no stopping them in their quest for a third successive title as they notched a 22-11 win.
Paul Warlow kicked 17 points for the visitors whose cause was given a boost by Narberth's indiscipline. The home side had a player yellow carded in each half and then had Nick Jones sent-off shortly before the end.
Llangennech kicked-off their campaign with a bonus point as they ran in four tries in a 35-13 home win over Dunvant and there was an excellent win on the road for Merthyr, who hung on to triumph 15-10 at Waunarlwydd.
Pontypool also came up with a try bonus point in the East as they ran in four tries in their 26-18 win at Blackwood. The visitors were 8-7 down at the break, but hit back to dominate the second half. Back row man Cai Treharne scored two tries for Pooler and wing Gavin Ferguson and centre Phil Price ran in the rest.
The students of UWIC, with five debutants in their ranks, beat Newport Saracens 15-3 at home, while two tries from Chris Perry steered Newbridge to a 30-13 win at Fleur de Lys.