Up front, loose head prop Keagan Bale and No 8 Harrison Keddie start in place of Harrison Walsh and Jon Fox respectively, while behind the scrum, scrum half Mike Hale starts ahead of Matthew Aubrey, Dan Jones moves from centre to outside half to replace the injured Luke Price and James Whittingham comes into the centre.
Lewis said, "There is no doubt we are bitterly disappointed to have lost in the final moments against England. Our defence was outstanding and we believe we deserved to come out of that game with a draw at least. However, everyone starting tomorrow's match has thoroughly earned their jersey from their performances in training.
"Our defence against South Africa will have to be at least as good as it was on Friday, as South Africa have a big set of forwards who constantly look to take the ball on.
"We will be looking to stick to our principals, challenging the opposition with ball in hand as much as possible.
"We must continue to concentrate on our performance and if that's good, the results will take care of themselves.”
Lewis feels strongly that experiences like this international tournament are beneficial on many fronts.
"This is the first rung of the ladder in terms of these boys becoming professional rugby players but they have responded well over the past fortnight, not only to a professional training and playing environment, but to the off field experiences that come with touring a foreign country too.” Wales also play France on Sunday.
Wales Under 18 v South Africa (Tuesday 19 August, 5.30pm South Africa time. The match will be shown in full on S4C at 10pm on Wednesday):
Morgan Williams; Shaun Pearce, Owen Watkin, James Whittingham; Elis Wyn Benham, Dan Jones; Keagan Bale, Liam Belcher, Kieran Assiratti, Seb Davies, Matthew Dodd, Tom Phillips (capt), Josh Middleton, Harrison Keddie.
Replacements: Dafydd Hughes, Harrison Walsh, Rupert Lewis, Jon Fox, Shaun Evans, Matthew Aubrey, Joe Gage, Jordan Roberts.
