And the 34-year-old openside, who will also equal Colin Charvis as Wales' most-capped forward, admits it will be a special occasion.
"It's a nice achievement and milestone to reach," said Williams, who be representing his country for the 94th time in the Friday-night clash at the Millennium Stadium.
"I did fear I would not play in the championship after receiving some criticism of late from Warren Gatland.
"But you have to take it on the chin at international level and respond in the right way on the field.
"I'm happy to get the call again and I want to put in a good performance," added Williams, who joins fellow Lions Mike Gibson, Jason Leonard, Willie John McBride, Rory Underwood and John Hayes, as well as Chris Paterson and Philippe Sella on 50 Championship appearances or more.
Williams has also paid tribute to grieving Wales team-mate Bradley Davies after the sudden death of his mother.
Davies, 23, will make his first Six Nations start just seven days after mum Cheryl died following a short illness.
British & Irish Lion Williams lost his own mum Julie to a lengthy battle with cancer in 2005 while younger brother Craig also died of the disease a decade ago.
And Williams has praised Blues colleague Davies for the way he has dealt with his own personal tragedy ahead of the Test clash in the Welsh capital.
"We have all supported him but I can't speak highly enough of Bradley.
"He is only 23 but he's shown he has an old head on young shoulders - he has not let it affect him in any shape or form on the training pitch.
"There is no doubt that it will be emotional for him when he runs out and then the anthem starts.
"I've been in Bradley's position before but he has been awesome this week.
"He had a couple of days off but, from experience, playing does take your mind off things and the sooner he came back in the better. It gives you something to focus on and I'm sure it will give his family a bit of rest as well. I hope Friday night will do that for them."