Japan won the 2017 Asia Rugby Championship on Saturday when they finished their campaign unbeaten following a 16-0 win over Hong Kong.

On a wet day at the Hong Kong Football Club, the Brave Blossoms won their 25th title out of a possible 30 and their 10th straight since it became an annual tournament.

But it was hardly a performance that will worry the sides drawn this week with Japan at the 2019 Rugby World Cup.

Once again the improvements made by Hong Kong under coach Leigh Jones were one reason for the tightness of the game in which Amanaki Lotoahea scored the only try.

"It's been a tough tournament, a lot tougher than last year," said Japan head coach Jamie Joseph. "The Hong Kong team has improved so much and they really tested us last week as well as this week so we are really proud of the win today."

The hosts were also helped to a certain degree by a plethora of errors from a Japan team that was admittedly a third or fourth-string side, with most of the big-name players currently playing Super Rugby.

The result marked the first time since 2008 -- bar 2015 when the same fixture was abandoned due to rain -- that the Brave Blossoms (who beat South Korea home-and-away and Hong Kong in Tokyo) failed to get a bonus point for scoring four or more tries.

"We are very happy with the results of both matches against Japan," said Dai Rees, Chief Rugby Operations Officer with the Hong Kong Rugby Football Union.

"We were able to put them under pressure, and that's something that we haven't been able to do in the past. Now it's important for us to follow up these two matches with wins against Korea in both Seoul two weeks from today and then back here again in June."

The game started with a plethora of turnovers with both defenses showing good line speed. As in Tokyo last week, the physicality of the Hong Kong pack served them well and they had the early edge in territory and possession.

And when Japan did get the ball, the visitors struggled to hold on to a wet, slippery ball.

The tightness of the game was reflected in the lack of scoring opportunities and the scoreboard operator had a quiet start to the evening, with both Jamie Hood and Takuya Yamasawa missing relatively simple kicks at goal.

However, the Japan flyhalf finally got his radar working in the 28th minute when he banged over a penalty from 43 meters out.

The visitors, however, failed to take full advantage as their line-out continued to struggle, not helped by an injury to Samuela Anise, and the teams turned around with Japan leading 3-0.

Yamasawa extended the lead shortly after the restart with a second penalty before adding the extras to Lotoahea's try in the 46th minute after the Japan wing had been put away by Kanta Shikao.

But with tenacious defense and errors to the fore, there was no further score until the 65th minute when Yamasawa banged over his third penalty of the day to seal the win and the title.