Teenage swimmer Quah Zheng Wen failed to qualify for the semi-finals of the 200m backstroke event on Wednesday.
Swimming in Heat 1, the 15-year-old, who is the younger brother of national swimmer Ting Wen, clocked a time of 2:03.45 seconds
The fastest qualifier for this event is Tyler Clary of the United States with his time of 1:56.24s.
His fellow American and hot favourite Ryan Lochte is close behind with 1:56.36s, with Japan's pin-up Ryosuke Irie third in 1:56.81s.
The world record for this event, 1:51.92s, is set by American Aaron Peirsol in 2009.
In the women's 100m freestyle event, Singapore's Mylene Ong clocked 56.33s to miss out on the semi-finals.
Swimming in Heat 3, Ong's time is also off her personal best of 55.70s.
China's Tang Yi led the way with a blistering time of 53.28.
Defending champion Britta Steffen of Germany, who holds the world record of 52.07s, only managed a time of 54.42s to scramble into the semis.