Samoa fly-half Tusi Pisi landed six penalties to give his side a slender 18-13 victory over Pacific Island rivals Fiji in Suva.
Pisi kicked five first-half penalties to give Samoa a 15-5 lead at halftime and added his sixth penalty, Samoa's only points of the second half, to finish with six goals for seven attempts.
The win lifted Samoa to the top of the three-team Pacific division in the six-nation tournament after their opening draw with Tonga.
Fiji had a win over Tonga and a loss to Samoa, while Tonga ended with a draw and a loss.
Japan have won the North American section with two wins ahead of the match later Saturday between the United States and Canada.
Three players received yellow cards in Saturday's match at Fiji's National Stadium, including Samoa captain David Lemi, his teammate Fautua Otto and Fiji's Leone Nakarawa.
Pisi punished Fiji errors with penalties in the second, 16th, 25th, 28th and final minutes of the first half. He missed one attempt in the 22nd minute of the second spell but landed another two minutes later to complete his team's scoring.
Fiji scored tries through center Nemani Nadolo in the 11th minute and winger Naplioni Nalaga in the 42nd. Nemani missed the conversions of both tries but added a second half penalty which briefly cut Samoa's lead to 15-13 before Pisi extended their lead with his final goal.
The defeat is a setback for Fiji ahead of next weekend's critical Oceania World Cup qualifying match against the Cook Islands. The winner of that match will take a place in Pool A at next year's World Cup alongside Australia, hosts England, Wales and another playoff winner.
The Cook Islands have never qualified for a World Cup but have a strong team, including players with Super Rugby experience.
Tries: Nadolo, Nalaga
Yellow Card: Nakarawa
Pens: Pisi 6
Yellow Card: Otto, Lemi
Fiji: 15 Metuisela Talebula, 14 Napolioni Nalaga, 13 Asaeli Tikoriotuma, 12 Nemani Nadolo, 11 Sireli Bobo, 10 Jonetani Ralulu, 9 Nikola Matawalu, 8 Nemani Nagusa, 7 Akapusi Qera (c), 6 Dom Waqaninburotu, 5 Leone Nakawara, 4 Api Ratuniyarawa, 3 Manasa Saulo, 2 Tuapati Talemaitoga, 1 Campese Ma'afu.
Samoa: 15 Faatoina Autagavaia, 14 Fautua Otto, 13 George Pisi, 12 Jonny Leota, 11 David Lemi (capt), 10 Tusiata Pisi, 9 Kahn Fotuali'i, 8 Taiasina Tuifua, 7 Jack Lam, 6 Maurie Fa'asavalu, 5 Daniel Leo, 4 Teofilo Paulo, 3 James Johnston, 2 Ti'i Paulo, 1 Logovi'i Mulipola.
Replacements: 16 Wayne Avei, 17 Anthony Perenise, 18 Census Johnston, 19 Piula Fa'asalele, 20 Iosefa Tekori, 21 Vavao Afemai, 22 Patrick Fa'apale, 23 Anitelea Tuilagi
Referee: Mathlieu Rayna (France)