The 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 her third goal of the Olympic tournament in just the fourth minute, turning in a cross from Kitty van Male, just moments after missing a nearly identical chance.
Ellen 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 fought back, 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.
In other matches later Tuesday, South Africa was playing New Zealand, Belgium was taking on China, hosts Britain faces South Korea, Germany was playing Australia and Argentina was playing the United States in a repeat of the final of last year's Pan American Games, which the U.S. won 4-2 in one of the biggest shocks in recent hockey history. That win earned the U.S. its ticket to the London Games.