Williams is rewarded for his fine hat-trick against Namibia on Monday, with Gatland recalling Lions star Roberts to his starting XV and dropping Jonathan Davies to the replacements bench.
Shane doesn't make the matchday 22 as Gatland decides not to risk the former IRB World Player of the Year following a thigh strain.
Fellow Lion Lee Byrne is selected at full back having started against Namibia but not been required against South Africa and Samoa, while Rhys Priestland returns to the fly-half berth ahead of Stephen Jones.
Gethin Jenkins retains his place in the front row having made his first start for any side in seven months against the Namibians, with Bradley Davies and Luke Charteris beating Alun Wyn Jones to the second row spots.
Former skipper Ryan Jones starts on the blindside flank in the absence of ankle injury victim Dan Lydiate who joins utility back James Hook on the sidelines.
Lloyd Williams, son of ex Lion Brynmor, has been given the nod ahead of Tavis Knoyle as Mike Phillips' scrum-half understudy following an impressive display from the bench last time out.
"Shane might have been available to us if we had pushed him, but he has taken a limited part in training this week and we are happy with the back three we have been able to put on the field for this game," said Gatland.
"Alun Wyn (Jones) and Jonathan (Davies) are two of the players who have started in all three games so far so we have freshened things up a little in the second row and the centre, but have both of them on the bench if we need them.
"In the front row Huw Bennett comes back in after resting up last week and Adam Jones also adds his extensive experience. Rhys Priestland, Mike Phillips, Jamie Roberts and George North are all also back in and we will be very much asking them all to continue where they each left off the last time they took to the field.
"Our focus is entirely on Fiji, we know if we can win one more game, and win it well, that will give us the best chance of reaching the quarter-finals of the Rugby World Cup with significant momentum."
Wales were knocked out of the 2007 World Cup by the Fijians and, although only Jenkins and Phillips remain from the side that lost out in Nantes, the Lions No9 would love to finally avenge that heartache.
"It would be nice to banish those memories," said Phillips.
"This is a completely different group but, personally, I was involved and it wasn't very nice. It was a low point. So it would be nice to get one over Fiji and put that game to bed."
Wales (vs Fiji, Hamilton, Sunday, 6am BST)
Lee Byrne; George North, Scott Williams, Jamie Roberts, Leigh Halfpenny, Rhys Priestland, Mike Phillips; Gethin Jenkins, Huw Bennett, Adam Jones, Bradley Davies, Luke Charteris, Ryan Jones, Toby Faletau, Sam Warburton (captain)
Replacements: Lloyd Burns, Paul James, Alun Wyn Jones, Andy Powell, Lloyd Williams, Stephen Jones, Jonathan Davies
NB Bold denotes Lion