Tonga recovered from an early yellow card to edge Samoa 16-12 in the first leg of the Oceania Rugby U20s Trophy in Apia.
The Kingdom were reduced to 14 players in the eighth minute when prop Sosaia Fale was sent to the sin bin for a reckless high tackle.
But six minutes later it was Tonga who opened the scoring with a try to left winger Lotu Inisi following a length-of-the-field move that started inside their own 22.
Samoa halfback Pupi Ah See dived over from close range in the 22nd minute to level the scores before Moo Moo Falaniko added the extras to give the Baby Manu a 7-5 lead.
Fullback Lisiate Folau restored Tonga's lead with a penalty goal to give the Kingdom a one-point buffer at half-time.
Malakai Uasi stretched the lead to six points, with just over ten minutes remaining, after capitalising on a poor clearance kick to beat three defenders en-route to the tryline with a blockbusting solo effort.
Samoa responded through substitute Fomai Fomai, who took a quick tap from a penalty and beat four defenders to score in the left hand corner and close the gap to one point but captain Simon-Peter Toleafoa was unable to convert the try to give the home side the lead.
From the resulting kick-off Samoa immediately coughed up possession after being penalised for not releasing the ball in the tackle.
Hateni Tafalo slotted the penalty kick from right in front to signal full-time and give Tonga a 16-12 advantage heading into Saturday's deciding second leg.
Tongan Captain Pita Halaifonua said it was a great start for his team.
"My team and I are so happy for this win and are also very grateful to the Samoan team for their effort today," he said.
"For the last match; come Saturday we want to play even harder and do our best in the final round.
"I would also like to thank all our Tongan supporters here in Apia and all those at home especially for our friends and family members - for their prayers for us and for their continuous support."
Goal-kicking proved to the downfall for Samoa, who failed to convert five of their of their six attempts with the boot.
Captain Simon Toleafoa said the team will go back to the drawing board and come back strong in the return leg.
"We were good in small patches but it just came down to the lower areas and points to goals that we could have got," he said.
"We will spread ourselves come Saturday to keep them tight. Also thank you for the fans for coming out today and sorry we didn't get the result but come Saturday we will work hard to get the win."
Tonga will qualify for next year's World Rugby Under 20 Trophy if they win the second leg or lose by three points or less.