Breakout Australian swimming star Ariarne Titmus backed up her 400m freestyle victory with another gold medal performance in a field that included Katie Ledecky, winning the 200m freestyle in an Olympic record 1:53.50.
Hong Kong's Siobhan Haughey, who led Titmus by a half-second at the 150 mark, took silver.
Titmus closed with a final split of 28.8 to pass Haughey with 15 meters to go.
Canada's Penny Oleksiak, a Rio 2016 gold medalist in the 100 free, took bronze as Katie Ledecky, never seeming to be in contention for the win, finished fifth.