Van Lamoen, who was Chile's flagbearer at the Opening Ceremony, was paired with the player she defeated for world gold last year - Georgia's Kristine Esebua - and was bundled out in straight sets.
Both players were well short of their best in the ranking round, hence lowly seedings of 31 and 34, but Esebua found her feet to see off Van Lamoen 6-0.
Esebua could not pull off another big win, though, and was soon defeated by second seed Lee Sung-jin of South Korea.
Lee missed the gold rings only once in nine arrows on her way to a dominant 6-0 victory over Samoa's Maureen Tuimalealiifano and added another four 10s as she racked up a four-set success against Esebua.
Lee's last-16 opponent will be Bishindee Urantungalag after two impressive wins for the Mongolian.
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