Tokyo - Japan are the darlings of the Rugby World Cup with some very unusual training methods - but none more bizarre than smacking their coach on the head with a plastic hammer.

The tournament hosts have been a revelation, storming to four straight wins to top Pool A and reach the quarter-finals for the first time.

But as they prepare for Sunday's clash with South Africa in Tokyo, video footage has emerged of veteran hooker Shota Horie whacking Japan coach Jamie Joseph in a game of rock, paper, scissors.

With the protagonists separated by a table, the loser has to grab a scrum-cap to cover his head before his opponent can reach for hammer and hit him.

Japan's heroes have been playing the "fast-twitch" games to improve hand-eye communication on their way to famous victories over Russia, Ireland, Samoa and Scotland.

Joseph's coaching staff are also writing daily brain teasers on the whiteboard at gym sessions to help stimulate the grey matter.

The hammer video, posted on Twitter by flyhalf Ryoto Nakamura, shows Horie landing a clean blow before Joseph wins the next round and gets a lick in himself - with so much force the hammer flies out of his hand.

"His one wasn't quite so playful," joked Horie, who won the decider to get in one last bash before raising his arms in triumph.

"But in the end, some Japanese skill win it for me."

Japan take on the Springboks in the quarter-finals in Tokyo on Sunday (12:15 SA time).

Teams:

Japan

15 Ryohei Yamanaka, 14 Kotaro Matsushima, 13 Timothy Lafaele, 12 Ryoto Nakamura, 11 Kenki Fukuoka, 10 Yu Tamura, 9 Yutaka Nagare, 8 Kazuki Himeno, 7 Lappies Labuschagne, 6 Michael Leitch (captain), 5 James Moore, 4 Luke Thompson, 3 Koo Ji-won, 2 Shota Horie, 1 Keita Inagaki

Substitutes: 16 Atsushi Sakate, 17 Isileli Nakajima, 18 Asaeli Ai Valu, 19 Wimpie van der Walt, 20 Amanaki Lelei Mafi, 21 Fumiaki Tanaka, 22 Rikiya Matsuda, 23 Lomano Lava Lemeki

South Africa

15 Willie le Roux, 14 Cheslin Kolbe, 13 Lukhanyo Am, 12 Damian de Allende, 11 Makazole Mapimpi, 10 Handre Pollard, 9 Faf de Klerk, 8 Duane Vermeulen, 7 Pieter-Steph du Toit, 6 Siya Kolisi (captain), 5 Lood de Jager, 4 Eben Etzebeth, 3 Frans Malherbe, 2 Bongi Mbonambi, 1 Tendai Mtawarira

Substitutes: 16 Malcolm Marx, 17 Steven Kitshoff, 18 Vincent Koch, 19 RG Snyman, 20 Franco Mostert, 21 Francois Louw, 22 Herschel Jantjies, 23 Frans Steyn