Olympic Games organisers have started rescheduling events as typhoon Nepartak was approaching the host city of Tokyo and expected to make landfall on Tuesday.
Weather authorities warned of heavy rains, gale force winds and high waves which could already affect the region from Monday onwards.
Rowing had all Monday events moved to Sunday, and Tuesday has also been cleared with events moved to Wednesday.
Other outdoor sports could follow as organisers are in close contact with sports federations about rescheduling and changing of the programme.
For now athletes were mainly dealing with high temperatures and humidity.
Tennis on Sunday applied its extreme heat rules with longer changeover breaks and a 10-minute break before the third set.