The Swedish duo leads after the first sub-sailing for the 470 class which is held off the small island of Enoshima a few miles southwest of Tokyo.
They immediately took the lead but initially had a tough fight against Australia.
Then Anton Dahlberg and Fredrik Bergström withdrew, but had to work to get ahead.
With cloudy weather, moderate southwesterly winds and waves of between one and two meters, the water offers some resistance.
Sweden is thus in first place, but several rounds remain for the men's 470 class who make a second round on Wednesday afternoon.
A total of ten races will be held in the basic round for 470 before the final is decided on Wednesday 4 August.
Sixth at the 2016 Olympics
The 470 duo on the men's side.
Anton Dahlberg and Fredrik Bergström, have had a successful year so far - most recently a World Cup gold in the 470 class in sailing in March, followed by the European Championship bronze in Vilamoura, Portugal in May.
In addition to the World Cup gold, the duo has previously won silver in 2017 and bronze in 2019 in the two-man dinghy class.
At the Olympics in Rio 2016, they ended up in sixth place.
As 470 and Nacra 17 made their entrance in the Olympics on Wednesday, all Swedish crews are now in competition.