Netherlands beat Belgium 3-0 in its first match on the electric blue pitch at Riverbank Arena. For Japan, it was the second defeat of the tournament after losing its opener 4-0 to host Britain.
Lammers scored in each half to bring her tournament tally to four goals and Ellen Hoog smashed in a powerful backhand shot to give the Netherlands a comfortable-looking 3-0 lead early in the second half.
But Japan fought back when Rika Komazawa, who had to leave the pitch in the first half after being hit in the face by a stick, scored in the 45th minute. Japan drew closer when Aki Mitsuhashi deflected a penalty into the Dutch net seven minutes later.
The Netherlands largely controlled play with a measured passing game, but Japan constantly put them under pressure and was dangerous on the break if it managed to force a turnover.
Lammers scored in just the fourth minute, turning in a cross from Kitty van Male, just moments after missing a nearly identical chance.
Hoog doubled the Dutch lead just before half time, turning away from a defender and giving Japan goalkeeper Sakiyo Asano no chance with her rising backhand shot.
Carlien Dirkse van den Heuvel went on a storming run down the left and cut the ball back for Lammers to easily score her second of the match early in the second half.
After Japan rallied, the Netherlands again asserted its control and Lidewij Welten nearly gave the Dutch a two-goal cushion with four minutes to go, but her shot rebounded safely off the post after beating Japan goalkeeper Sakiyo Asano.
Hoog said there was room for improvement with her team.
"We are satisfied with the two wins and six points, but today we were not very good so we have things we can work on," she said. "We were not playing together and as a team, and we were not very sharp."
Komazawa said her team did not get balance its game plan well.
"The tactic in the first half was to be solid in defence because Holland are stronger," Komazawa said. "We were too defensive though and should have attacked more."
In other matches later Tuesday, first-time Olympians Belgium held silver-medallists China to a goalless draw to win their first point of the tournament; and New Zealand beat the South African team 4-1 to move into first position in Pool B.
Kiwi captain Kayla Sharland scored two field goals, while defender Clarissa Eshuis and Charlotte Harrison — who celebrates her 23rd birthday today — both scored from penalty corners.