Williams' Test ambitions go on the line against a fired-up Otago side, having witnessed a superb display from England World Cup winner Neil Back - his main rival for the Test number seven shirt - during Wednesday's victory over Wellington.
Lewis Moody would also have been in the mix, but his troublesome knee injury forced him out of contention before the Lions left their Christchurch base, handing Ireland's Simon Easterby a second start in four days.
So it would appear to be a straight fight between Williams and Back for the coveted openside flanker's role, as Lions chief Sir Clive Woodward moves closer towards announcing his Test team.
Williams has already experienced defeat on the 11-game tour, having started against New Zealand Maori last weekend, and he knows that in terms of first Test recognition, this encounter means everything.
"It didn't go well for us against the Maori, but as a team and individuals, we've just got to learn from it," he said.
"There is no point in moping about it, you've just got to try and brush yourself down and carry on.
"In terms of the first Test, it is the last chance - Clive more or less told us that.
"Neil Back is a world-class player, and Lewis (Moody) has played well in the games he's had. There are a lot of boys who have only played one game, if any, so you have got to feel for those guys.
"A lot of us are getting a second chance to try and prove to ourselves, as much as anyone else, that we can play. I will be amazed though, if the starting XV for the first Test is the same that plays in the third Test. Through injuries and form, things change.
"And that is what you've got to get your head around. Even if you have slipped back a bit for the first Test, you can't afford to go off tour because things can happen at the last minute, so you gave got to be professional and keep going," added Williams.
Back, almost seven years Williams' senior, stole a march with his all-action performance against Wellington, leaving the Welsh international under no illusions that he must produce the goods this weekend.
"Taking the Maori on so early in the tour was always going to be difficult. We are still finding our feet together and trying to gel. We were desperate for a win on Wednesday, and momentum is slowly gathering," he said.
"It's not like the Australia tour four years ago when, let's be honest, there were a couple of easy games in between the bigger games. It's not like that this time."
Otago, who beat the Lions in 1908, 1950, 1959, 1966 and 1993, will be without All Blacks squad members Carl Hayman, Anton Oliver and James Ryan, but they still possess sufficient quality to seriously test the tourists in what could be a ferocious physical battle.
Teams for Otago versus the British & Irish Lions, Carisbrook, Dunedin, kick-off 0810 (BST).
Otago: G Horton; H Pedersen, N Brew, S Mapusua, M Saunders; N Evans, D Lee; C Hoeft, J Macdonald, C Dunlea, F Levi, T Donnelly, C Newby (capt), J Blackie, G Webb.
Replacements: J Vercoe, J Aldworth, A McClintock, A Soakai, C Smylie, R Bambry, J Shoemark.
Lions: G Murphy (Leicester and Ireland); D Hickie (Leinster and Ireland), W Greenwood (Harlequins and England), G D'Arcy (Leinster and Ireland), S Williams (Neath-Swansea Ospreys and Wales); C Hodgson (Sale Sharks and England), C Cusiter (Borders and Scotland); G Rowntree (Leicester and England), G Bulloch (Glasgow and Scotland, capt), M Stevens (Bath and England), S Shaw (Wasps and England), D O'Callaghan (Munster and Ireland), S Easterby (Llanelli Scarlets and Ireland), M Williams (Cardiff Blues and Wales), R Jones (Neath-Swansea Ospreys and Wales).
Replacements: S Thompson (Northampton and England), A Sheridan (Sale Sharks and England), D Grewcock (Bath and England), M Owen (Newport Gwent Dragons and Wales), M Dawson (Wasps and England), O Smith (Leicester and England), R O'Gara (Munster and Ireland).
Referee: Lyndon Bray (New Zealand). Touch judges: Paul Honiss and Kelvin Deaker (both New Zealand).