Japan may have maintained its impressive unbeaten record in the Asia Rugby Championship by defeating Hong Kong 29-17 on Saturday, but it was hardly a classic performance.

With the eyes of the rugby world on Japan ahead of next week's draw in Kyoto for the 2019 World Cup, the Brave Blossoms were made to work quite hard for the bonus-point win at Prince Chichibu Memorial Rugby Ground as the visitors forced the men in red and white into a plethora of errors.

Hong Kong has had a professional program for its top 15s players for just 18 months, but the benefits are already obvious as they took on Japan in the physicality stakes and stopped the hosts from playing their usual high-speed game.

"I take my hat off to Hong Kong," said Japan coach Jamie Joseph. "They put us under a lot of pressure and stopped our game and we took time to react."

Second-half tries from 2015 Rugby World Cup veterans Akihito Yamada (his second of the game), Shota Horie and Harumichi Tatekawa eventually saw Japan home after it trailed 17-12 with the game entering the final 20 minutes.

"From a pure coaching perspective I am bloody annoyed with the last 20 minutes," said Hong Kong coach Leigh Jones. "I think I got my substitutions wrong and it may have cost us a rare victory in Japan."

It had all started so easily for Japan when Kanta Shikao's clever grubber kick — his first touch of the ball in test-match rugby — saw Shuhei Matsuhashi go over for the opening try in the second minute.

But there were early signs Japan were struggling with the interpretation of referee Tim Baker, and the sin-binning of Samuela Anise in the 16th minute allowed Hong Kong back into the game.

Jamie Hood banged over a penalty and then Yiu Kam Shing made the most of a good inside pass from Matt Rosslee to go over between the sticks as the visitors went 10-7 up.

Yamada's first try was, courtesy of some good work by Naohiro Kotaki in regaining a spilt ball, saw Japan go into the break with a slightly fortuitous 12-10 lead.

The advantage didn't last long though, as Rosslee again showed his class. His powerful run and clever offload saw Tyler Spitz restore Hong Kong's lead.

With both coaches emptying the bench, it was Japan which ended strongest with its veteran trio making sure the Brave Blossoms need just a bonus point in next week's return game in Hong Kong to lift the title.

"That was exactly what we needed," said Joseph. "It brings the players a wee bit of reality of what is expected at this level of rugby.

"It was a match when we could have easily lost our shape and control. But we did well to keep our discipline and focus and I'm proud we did not lose our way."

For Japan, Ryuji Noguchi had another solid game at fullback, while Shikao impressed on debut, the center having to finish the game at flanker as injuries took their toll.

Hong Kong, meanwhile, was left to rue what might have been.

"I'm disappointed we didn't get the win after leading with 20 minutes to go and I'm disappointed we couldn't make a bonus point," captain Ben Roberts said.