Tonga need a win over Samoa on Sunday to reach the World Cup semi-finals, a stage they have not made since 2017.
Tyson Frizell, 35, is part of the side's campaign and is the only player to have represented three nations at a World Cup.
He played for Wales in 2013, then was part of Australia's victorious team in 2017. His father was born in Wales, while his mother's heritage is Tongan.
Frizell said the current tournament would be his last. "I'm in the moment now and it means everything to me now. It's definitely going to be my last one and I want to play as well as I can to give ourselves the opportunity to make history and do something special."
After the World Cup, he will join Wests Tigers for the 2027 season, following 306 NRL games for Cronulla, St George Illawarra and Newcastle.
Tigers coach Benji Marshall praised Frizell's preparation after the signing was confirmed. "He's the most professional player I've ever seen when it comes to preparation," Marshall said.
Frizell said joining the club was "a big challenge" and that he wanted to earn the players' respect and perform well.
He said accepting Marshall's offer was an easy decision, adding: "It is nice to be well respected and having someone wanting you to be a part of their club because they think you can add some value to it."