After being overlooked by Wales during the RBS 6 Nations, Williams was handed a World Cup lifeline when he was selected in Warren Gatland's preliminary 45-man squad which will later be reduced to 30.
But the 35-year-old, who has toured Australia (2001), New Zealand (2005) and South Africa (2009) with the Lions, was warned by Gatland that he would have to fight for a place at the World Cup. And Williams admits he wouldn't want it any other way.
"Warren has been spot on in what he has said," said Williams.
"He doesn't want to dish caps out, he wants you to earn them and I totally agree with that.
"To get 100 caps would be the ultimate for me, but if I don't, I don't. I would have taken one cap.
"I look at it as the glass being half-full. I've had a great career, and if I manage to get two more, then happy days.
"We get picked on who the coaches think are on form and what suits their side. I suppose the slate is clean for everyone."
Williams, who lined-up against Wales last weekend as the Barbarians recorded a 31-28 victory, may have fallen behind Sam Warburton in the pecking order at both club and country but he reckons his vast experience could play a decisive factor.
"Obviously that's one tick in my box; experience," said Williams, who has signed a one-year contract extension with the Cardiff Blues.
"Although we have got a lot of young players we've got a lot of experience as well. If I can add to that then I will be more than happy."
Wales begin their preparations for the global tournament on Monday and following two training camps in Poland they will play three World Cup warm-up matches before Gatland reduces the squad.
"Obviously it's a long summer of training ahead and the three pre-World Cup games," said Williams.
"When you are 35 and you have played the amount of rugby I have, you are not going to be as sharp as what you used to be.
"But I've been happy all year. My form I think has been pretty good."