Lebanon coach Michael Cheika said his side deserved better from the 36-0 loss to Tonga, but fatigue proved costly late in the match.
"I didn't want it to finish with that scoreline, I thought we merited better, but fatigue played as a factor at the end," Cheika said.
He added: "I'm super proud of the boys. I don't say that lightly. Some of them are part-time footballers and they handled those conditions pretty well across the board."
Lebanon had stayed error-free in the first half and created the better early chances, but could not convert them.
Tonga made 13 errors, yet took advantage of key opportunities early before scoring four tries in the final 10 minutes.
Wingers Sione Katoa and Jojo Fifita each crossed twice.
The win came six days after Tonga's gritty victory over England in Perth, with playmaker Isaiya Katoa, centre Robert Toia and Tyson Frizell unavailable.
Tonga now face Toa Samoa in Parramatta next weekend, with a semi-final place still not secured and their unbeaten Rugby League World Cup run at stake.
Samoa first meet Papua New Guinea in Port Moresby on Saturday, then return to Australia for the contest.
Tonga captain Jason Taumalolo said the match carried significance for both nations and would be physical.
"It's a huge game for both countries given the history between Tonga and Samoa. We're expecting another physical game and we're looking forward to it," Taumalolo said.
Despite Tonga's 28-point loss to Samoa in last year's Pacific Championship, revenge is not their focus.
Taumalolo said: "I've been lucky enough to be part of some big games, and next Sunday is going to be a big occasion. I'm sure there will be a few nerves but I'm confident in the team we have which will make for a great game."
Lebanon will conclude its World Cup campaign against France next Sunday afternoon.