Takashi Kikutani and Takehisa Usuzuki's second-half tries powered Japan to a 20-14 win over the United States on Sunday in a RWC warm-up.
John Kirwan's side dominated the game with their physical play in the second half, shutting down the States to secure the confidence-boosting victory.
Playing before a partisan crowd at Chichibunomiya Stadium, Japan scored first on a try by Taihei Ueda six minutes into the contest, which began in a driving rain. Ueda took a pass from Alisi Tupuailai, who set up the score well.
Kevin Swiryn responded for the USA seconds later with a try to level the scores at 5-5. The States moved ahead 7-5 on the conversion by Nese Malifa but Japan retook the lead with a drop-goal from Ryan Nicholas in the 12th minute.
Paul Emerick then scored a try on a long uncontested run to give back the advantage to the Americans just before half-time. Swiryn added the conversion to make it 14-8 at the break.
In the second-half, Kikutani barreled through the American defence from just outside the try-line for a try in the 53rd minute to move the hosts within a point. Shaun Webb put Japan back on top 15-14 with the conversion.
Usuzuki extended Japan's winning margin to 20-14 with a try in the 66th minute.
Japan, who lost to Italy 31-24 in Cesena last week, made twelve changes for the match. USA coach Eddie O'Sullivan made eight changes to his side after losing 27-7 to Canada in Glendale, Colorado, last weekend.
Japan will face France in its opening Pool A World Cup match on September 10, while the USA begin their campaign in Pool C against Ireland the day after.
Tries: Ueda, Kikutani, Usuzuki
Tries: Swiryn, Emerick
Con: Malifa, Swiryn
Japan: 15 Taihei Ueda, 14 Takehisa Usuzuki, 13 Alisi Tupuailai, 12 Ryan Nicholas, 11 Hirotoki Onozawa, 10 Murray Williams, 9 Tomoki Yoshida, 8 Takashi Kikutani, 7 Tadasuke Nishihara, 6 Sione Vatuvei, 5 Luke Thompson, 4 Hitoshi Ono, 3 Nozomu Fujita, 2 Yusuke Aoki, 1 Naoki Kawamata.
Replacements: 16 Shota Horie, 17 Kensuke Hatakeyama, 18 Toshizumi Kitagawa, 19 Itaru Taniguchi, 20 Atsushi Hiwasa, 21 Koji Taira, 22 Shaun Webb.
USA Eagles: 15 Blaine Scully, 14 Kevin Swiryn, 13 Tai Enosa, 12 Paul Emerick, 11 James Paterson, 10 Nese Malifa, 9 Mike Petri (c), 8 JJ Gagiano, 7 Louis Stanfill, 6 Patrick Danahy, 5 Hayden Smith, 4 Scott LaValla, 3 Eric Fry, 2 Phillip Thiel, 1 Mike Macdonald.
Replacements: 16 Chris Biller, 17 Shawn Pittman, 18 Nic Johnson, 19 Ryan Chapman, 20 Tim Usasz, 21 Roland Suniula, 22 Andrew Suniula.
Referee: Jérôme Garces (France)