Samoa defeated Japan 31-21 to secure their third consecutive Cup quarterfinal at the Hong Kong round of the World Rugby Sevens Series.

The Manu Samoa 7s will play South Africa at 9:22pm (Samoan time) in their final game of day two to determine who finishes top of the pool.

They had a perfect start to the game against Japan with John Vaili executing another of his trademark sky-high kickoffs.

Alamanda Motuga caught the ball and sped away to score one of the fastest series tries after just 8 seconds of play.

The Manu stretched their lead out to 12 within three minutes when Melani Matavao scored off the back of a great bust from Laaloi Leilua.

Japan replied through Kameli Raravou Soejima after some nice offloading, but Leilua had something special in his locker for his second game back in the Manu Samoa 7s since his last tournament in 2018.

The centre chipped and chased, soccering the ball forward twice to score and make it 19-7 with a minute left in the half.

But a nice run from Soejima set up Siosifa Lisala to score for Japan, bringing them within five points at halftime.

Tomasi Alosio made a good break to open play up early in the second half.

Tofatu Solia got the ball out wide after Samoa shifted the point of attack, and brushed off three would-be tacklers to score.

Solia saved a try with his defence on the other end, knocking Lisala into touch after a dangerous run down the sideline.

A poor Samoa throw at the lineout undid that good work though, and Katsuyuki Sakai took advantage of the overthrow to score a simple try and make it 26-21 with three minutes left.

Vaili started another attack for Samoa with a nice step and line break before offloading to Alosio to keep the play alive.

It looked like Samoa had missed their chance, but the ball was finally sent wide to Solia to score his third try of the tournament with one minute remaining.

Samoa shifted into possession retention mode, keeping a hold of the ball well while still threatening on attack until the final whistle.

Motuga and captain David Afamasaga deserve a mention for their excellent work in the ruck, helping Samoa control the breakdown throughout the game.