Australia's Emma McKeon broke the Olympic record in the women's 100m freestyle preliminary heats on Wednesday with a time of 52.13 seconds.
McKeon broke the 52.62 set by Sarah Sjostrom, and the time was also a new personal best for the Australian.
Hong Kong’s Siobhan Haughey, who won silver in the 200m freestyle earlier on Wednesday, was second quickest in 52.70 ahead of Great Britain's Anna Hopkin and Australia's Cate Campbell.
Great Britain's Luke Greenbank was fastest in the men's 200m backstroke heat on 1:54.63 ahead of Russian Evgeny Rylov, one of the favourites for the gold medal.
American Bryce Mefford was third ahead of South Korea's Lee Juho.
The silver medalist in the women's 100m backstroke, Tatjana Schoenmaker, was fastest in the 200m backstroke heat posting a time of 2:19.16, with American Lilly King second quickest.
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