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Wales, pipped 17-16 by the Springboks in Wellington, need to to beat the Pacific islanders to have a chance of qualifying for the quarterfinals.
Gatland made just one change to the reserves, bringing prop Gethin Jenkins in to replace Ryan Bevington on the bench.
Samoa coach Titimaea Tafua made one change to the starting side that beat Namibia 49-12 in Rotorua on Wednesday, with Ofisa Treviranus replacing the injured Taiasina Tuifua at blindside flanker.
Wales: 15-James Hook, 14-George North, 13-Jonathan Davies, 12-Jamie Roberts, 11-Shane Williams, 10-Rhys Priestland, 9-Mike Phillips, 8-Toby Faletau, 7-Sam Warburton (captain), 6-Danny Lydiate, 5-Alun Wyn Jones, 4-Luke Charteris, 3-Adam Jones, 2-Huw Bennett, 1-Paul James
Replacements: 16-Lloyd Burns, 17-Gethin Jenkins, 18-Bradley Davies, 19-Andy Powell, 20-Tavis Knoyle, 21-Scott Williams, 22- Leigh Halfpenny
Samoa: 15-Paul Williams, 14-Sailosi Tagicakibau, 13-George Pisi, 12-Seilala Mapusua, 11-Alesana Tuilagi, 10-Tusi Pisi, 9-Kahn Fotuali'i, 8-George Stowers, 7-Maurie Faasavalu, 6-Ofisa Treviranus, 5-Kane Thompson, 4-Daniel Leo, 3-Anthony Perenise, 2-Mahonri Schwalger (captain), 1-Sakaria Taulafo
Replacements: 16-Ti'i Paulo, 17-Census Johnston, 18-Joe Tekori, 19-Taiasina Tuifua, 20-Jeremy Sua, 21-Eliota Sapolu Fuimaono, 22-Tasesa Lavea