Williams scored a try and created another two as his hometown club dug deep to edge a pulsating affair with man-of-the-match Joel Foster kicking 20 points from the tee.
It took Williams just nine minutes to mark his return to the home of Welsh rugby with a try, racing away unscathed behind the uprights. That was the wing wizard’s first try at the stadium since December 2011 when he scored in his final international against Australia.
But Caerphilly marched back immediately with a classy team effort.
Full-back Tom Candy weaved his way through from halfway before finding former Pontypridd centre Matthew Nuthall – a multiple National Cup winner – who fed Chris Thomas.
And the Cheesemen went ahead on 18 minutes when outside-half Matthew Hurley bruised his way in at the posts and converted.
Williams almost wriggled his way through only to be halted by a charging Nuthall.
Amman looked dangerous going forward and the Caerphilly defence could only hold up for so long. Doug Jenkins spilled the ball on the try-line for the Reds who had to settle for a Foster penalty.
It was not long before Williams was at it again and he lent a hand in their second try, teeing up his brother-in-law Gavin Lewis in the corner.
But Caerphilly bounced back before the break with a storming solo effort from Gareth Dare.
Candy’s outrageous pass gifted the wing space who brushed aside a number of tackles – including two from Williams – to slide under the sticks.
It looked as if the Green and Blacks would lead at the interval, but Williams was involved again in Amman’s next try, fizzing the ball to lock Shaun Cadell who slid in the corner. The score was given via the TMO.
After half time, Foster and Hurley exchanged penalties to keep things on a knife edge.
Foster added penalties on 50 and 57 minutes to hoist his team ahead by five points.
But Caerphilly refused to back down, marching up the other end with Nuthall – who scored in Pontypridd’s 26-25 Cup win in 2005 – going over and Hurley kicking the conversion for a narrow two-point advantage.
However, the lead lasted just a matter of minutes when Foster added three points after Caerphilly were penalised for offside.
Josh Hayward departed to the sin bin for going in off his feet which gifted Foster another penalty from the tee.
And Amman wrapped things up with five minutes to play with Chris Rees crossing in the corner.
Amman require a win next week in Division Three West B to clinch the title and seal a league and cup double and complete what would be a season to remember.
