With 98 caps and two Grand Slams behind him, the Cardiff Blues flanker has plenty of experience to call upon as he bids to reach his fourth World Cup.
"Look, when I was left out the Six Nations squad last season I thought that was me done and dusted," said Williams.
"But then I was pulled into the World Cup training squad of 45 and I've had a good summer's training and been given the opportunity again. Everybody gets frustrated watching and so, yes, I am glad to get a run.
"For a few of us who haven't had a chance, there is the incentive that, with things still up in the air, a good performance will help. But, when you get to the stage of your career I am at, then, even though the competitive edge is still there, you have to be realistic.
"I have trained really hard, I am in the best shape I've ever been, and that makes you want to play even more.
"There are quite a few of us who haven't played all summer and who are going to have to go straight in on Saturday and hit their straps immediately. It's not going to be easy, but that's just how it is. You have to deal with it.
"The squad has become really tight. Everyone has helped and encouraged each other, and so to have to drop out will be tough to say the least. But they can only take 30 to New Zealand, and that is professional sport. All I would say is that the people who aren't in the 30 need to keep their heads up."
Wales v Argentina, Millennium Stadium, Saturday 20 Aug, 2011 (KO 2.30pm)
WALES:Lee Byrne; Leigh Halfpenny, Jonathan Davies, Jamie Roberts, George North; James Hook, Tavis Knoyle; Paul James, Richard Hibbard, Adam Jones, Bradley Davies, Alun Wyn Jones, Dan Lydiate, Martyn Williams (captain), Andy Powell.
Replacements: Huw Bennett, Ryan Bevington, Jonathan Thomas, Justin Tipuric, Lloyd Williams, Stephen Jones, Aled Brew
Argentina 15 Martin Rodriguez, 14 Horacio Agulla, 13 Marcelo Bosch, 12 Santiago Fernandez, 11 Gonzalo Camacho, 10 Felipe Contepomi (c), 9 Nicolas Vergallo, 8 Juan Fernandez Lobbe, 7 Juan Manuel Leguizamon, 6 Julio Farias Cabello, 5 Patricio Albacete, 4 Manuel Carizza, 3 Juan Figallo, 2 Mario Ledesma, 1 Rodrigo Roncero.
Replacements: 16Agustin Creevy, 17 Martin Scelzo, 18 Mariano Galarza, 19 Alejandro Campos, 20 Agustin Figuerola, 21 Nicolas Sanchez, 22 Juan Imhoff