Lammers, who by a quirk of selection and injury is making her debut at the Games aged 31 despite being an international for nine years, was again on top form.
It took just four minutes for her to add to the two goals she scored against Belgium, converting a good pass from Kitty van Male.
Ellen Hoog doubled their lead just before half-time and when Lammers made it 3-0 in the 43rd minute it seemed the game was as good as won.
It was almost a carbon copy of what their men did yesterday, as they squandered a 2-0 lead to India before eventually winning 3-2.
But by preventing any further Japanese breakthroughs Netherlands made it two successive wins to set the early pace in Pool A.
However, wins for either China - against lowly Belgium - or Great Britain, who face Korea, will knock the Netherlands off top spot.