Bristol Bears tighthead prop Nick English replaces Ben Warren while on the other side of the scrum Dragons’ Tom Devine comes in for Kemsley Mathias.
Scarlets lock Morgan Jones earns a starting spot after impressing from the bench in Saturday’s defeat to France, replacing Ed Scragg.
In the backline, hard running winger Deon Smith replaces Ryan Conbeer while Leicester Tigers’ Sam Costelow gets a chance to impress from the outset at inside centre with Max Llewellyn dropping down to the bench.
It will be the first time Wales have played at the venue with their two previous outings held in Rosario where a victory and loss have been recorded against Argentina and France respectively.
“We have taken lessons from our defeat to France, but just as important has been the refreshing mindset the players have displayed since Saturday and we’re all now looking forward to Fiji,” said Williams.
The South Sea Islanders may have been on the receiving end of two defeats against Argentina and France, but their flair has clearly been in evidence with some brilliant ball handling and well taken tries.
“Fiji have played some excellent stuff this tournament, and have been a challenge early on for both France and Argentina,” added Wiiliams.
“Therefore returning to the focus of the first two games is key for us. We cannot lose sight that winning games has been a focus of ours coming to this World Cup and we have three more tough challenges to do that.”
U20 World Rugby Championship
Wales U20 v Fiji U20
Club de Rugby Ateneo Inmaculada, Santa Fe
12 June, KO 1pm UK
15 Ioan Davies (Cardiff Blues)
14 Tomi Lewis (Scarlets)
13 Tiaan Thomas-Wheeler (Ospreys)
13 Sam Costelow (Leicester Tigers)
11 Deon Smith (Dragons)
10 Cai Evans (Ospreys)
9 Dafydd Buckland (Dragons);
1 Tom Devine (Dragons)
2 Dewi Lake (Capt – Ospreys)
3 Nick English (Bristol Bears),
4 Morgan Jones (Scarlets)
5 Jac Price (Scarlets)
6 Iestyn Rees (Scarlets)
7 Tommy Reffell (Leicester Tigers)
8 Jac Morgan (Aberavon/Scarlets)
Reserves: Will Griffiths (Dragons), Garin Lloyd (Ospreys), Rhys Davies (Ospreys), Kemsley Mathias (Scarlets), Ben Warren (Cardiff Blues). Ed Scragg (Dragons), Lennon Greggains (Dragons), Harri Morgan (Ospreys), Ryan Conbeer (Scarlets), Max Llewellyn (Cardiff Blues), Rio Dyer (Dragons)
* Dragons centre Aneurin Owen has been forced to withdraw from the tournament after suffering concussion in the opening minutes of the first game against Argentina. His place has been taken by Cardiff Blues No 8 Ioan Rhys Davies who adds back row cover to the squad.