Williams has recovered from a shoulder injury to return to action just a week away from the opening international against world champions South Africa in Durban.
And the 33-year-old believes it is now or never if he is to beat fellow Magners League veteran David Wallace to the Lions' No7 Test shirt.
"For a lot of us, Saturday's game is more or less like a final trial. We want to make it as hard as possible for Geech and the other coaches next week when they pick the Test team," said Williams.
"Geech made it clear from Day 1 that the Test side won't be selected until after Tuesday's match and that gives all of us something to aim for. We've got to go out on Saturday and raise the standard set by the team on Wednesday. I don't think there will be a danger of anyone trying too hard, it is just a question of playing your normal game.
"I know I've probably lost a bit of ground because of my injury. It has been frustrating watching the guys play and train, but now I've got to make the most of my chance."
Despite having been on two previous Lions tours, Williams has only seen six minutes of Test action for the world's most-famous touring outfit. At one stage last week it appeared as though his chances of adding to that tally were gradually receding but now he insists he is fully fit and raring to go.
"I was alright during the game against the Royal XV, but on the day after the match I couldn't lift my arm above shoulder height. It's tough on tour when you are not playing and can't even train.
"I've trained all week and the shoulder has been as good as it has been for a long time. They hoped it would have cleared up for the Cheetahs match but they put a cortisone injection in to get the inflammation down and that seems to have done the trick."
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