"What has gone on before will be put to one side when the early games start - it will become irrelevant," said Williams, who has already toured with the Lions in 2001 and 2005.
"The back row is the most competitive position on the tour and good players like Ryan Jones, Tom Croft and Denis Leamy have all missed out.
"I was the favourite for a starting Test spot four years ago. There will be four or five certainties but it is wide open for the rest of the players to fight for a spot in the team.
"The team that will be selected for the first Test will be those players who impress in the opening games."
If the Cardiff Blues star is to earn a Test spot this time around, he will have to fight off the challenge of Ireland's David Wallace and England's Joe Worsely, both of whom had impressive Six Nations campaigns for their respective countries.
With just one Test cap to his name from his previous two tours, Williams admits that he starts as underdog to Wallace when it comes to adding to that tally in 2009.
The Munsterman has been in stunning form for both province and country so far this season and Williams expects his 32-year-old rival to start as favourite for a Lions starting spot.
"David is in pole position for the Test team because he has been awesome for Munster and Ireland this season.
"There is fierce competition for the seven spot including Joe Worsley. But David deserves that recognition along with the other Irish players because of their Grand Slam success."
Relief may have been the over riding emotion on hearing about his selection for a third successive tour but, like so many other Lions squad members, Williams' attention has already turned to being part of a winning Test series.
"It was a great relief to be selected and feels just as special as eight years ago," added Williams, who made his solitary Lions Test appearance in the final match of the 2005 tour of New Zealand.
"I'm so glad to be on the tour but the aim is to win the series.
"We will only have a couple of weeks before the Test series starts so we'll have to gel quickly.
"Only four successful Lions teams have won in South Africa so to be part of a winning tour would be really special."