For several hours on Thursday, Laurentia Tan was third in the para-equestrian dressage standings, on course to repeat her London 2012 bronze-winning feat in the championship test (Grade 1a).
But by late afternoon, the 37-year-old, who has cerebral palsy, dropped to fifth on 73.522 points - 0.304 behind third-placed Sergio Olivia (73.826) of Brazil.
Britain's Sophie Christiansen (78.217) and Anne Dunham (74.348) clinched gold and silver respectively in this event, which is meant for athletes with the most severe impairments.
Singapore's other rider Gemma Rose Foo, 20, finished 24th out of 26 riders on 62.000.
Tan said she was "a little disappointed" with her ride. Through a sign language interpreter, she said: "I felt the horse could've been a little bit more active in the ride, it was a little bit too relaxed."
With four medals from the last two Paralympics, Tan was tipped to be on the podium. She added: "Anything can happen with this sport. Not having a podium finish will not affect my mood for tomorrow, because tomorrow is a new day."
SINGAPOREANS IN ACTION
Swimming: Women's 100m freestyle S5 heats (10.04pm) & final if she qualifies (tomorrow, 6.29am)
Athletics: Women's 100m T52 finals, 10.05pm
Foo was not entirely satisfied with her ride either, citing a lack of strength post-surgery and the distraction of windy conditions.
She was thrown from her horse in March this year and had to undergo surgery to repair her ruptured spleen. "I was calm, but it was the wind factor. I did the best I could," she said.
Both Tan and Foo were also competing in the individual freestyle test yesterday.
Singapore sailors Jovin Tan and Yap Qian Yin withdrew from the two-person keelboat (Skud 18) event on Thursday after Yap, 26, reportedly suffered a seizure.
A statement from the Singapore National Paralympic Council said: "While (Yap) is currently stable and recovering, the doctors advised against her participation in the event on Sept 15.
"The health and well-being of our athletes take priority."
The duo had completed six of the 11 scheduled races and were ranked ninth out of 11 teams.