Having already beaten the Japanese 3-0 on the way to the final of the Champions Trophy in Rosario earlier this year, Kate Walsh’s side were in control from start to finish, bettering that result by one.
Two goals from Alex Danson, and one each from Sarah Thomas and Sally Walton, secured Team GB’s first win of the pool – one which contains World Cup runners-up the Netherlands.
The only negative on a great opening night was the loss of captain Walsh, who left the field late on after suffering a dreadful blow to the face from a wayward stick.
Great Britain flew out of the traps showing no sign of early nerves as Georgie Twigg, the youngest player in the team at 21, fired narrowly over after a good early run.
Danny Kerry’s team got off the mark when Reading’s Danson found the net in the seventh minute breaking a number of tackles to hammer home with a reverse stick.
The Japanese goalkeeper pulled off a great save from a 20th minute penalty corner as Team GB controlled the first period.
And Great Britain, who came sixth last time around in Beijing, doubled their advantage shortly after when Hannah Macleod lifted the ball in for Wales’ Thomas who deflected home.
Bowdon Hightown defender Walton made it three from a Twigg assist, flicking in after Japan failed to clear from a penalty corner.
Three-time Olympian Helen Richardson then supplied a great reverse pass inside the area, and Danson finished with assurance to put Team GB further clear just before the interval.
Goalkeeper Beth Storry had very little to do throughout as Japan struggled to get a foothold in the contest, but was solid on the rare occasions she was called upon.
Danson had the chance of a first-half hat-trick but lifted over when put through once more.
Crista Cullen rapped the right hand post from a penalty corner in the second period, but Team GB seemed content to maintain their significant first-half advantage.
Richardson, the second most-capped player in the squad, joined Walsh on the sidelines just before the end after being struck directly on her right knee by the ball.
Japan improved after the break but Team GB were always in control and will be looking to maintain their good start when they take on South Korea on Tuesday in their next Pool A clash.