Rainfall is expected to peak on Saturday morning and continue to batter Taiwan until Saturday evening, the Central Weather Administration said.
Taiwanese authorities have urged people to stay away from coasts and mountains.
Work and classes were suspended in nearly all of Taiwan's 22 counties and cities on Friday - what's colloquially known as a "typhoon holiday" - as Champi approached the island.
Taiwan's $5tn (£3.7tn) stock exchange was also closed on Friday because of the severe weather.
Only Kinmen county, an archipelago located to the west of Taiwan's main island, continued work and classes as usual.
In Japan, power cuts were reported in nearly 8,400 households in the southern Okinawa prefecture, according to the local power company.
More than 160 flights have been cancelled across four Japanese airlines.
Champi has also stirred up heavy rains and winds in Batanes, a Philippine island chain below southern Taiwan.
Photos posted on the local government's Facebook page showed waves crashing into seaside villages and roads closed to traffic. There were no immediate reports of casualties.
Typhoon Champi became the strongest storm of the year earlier this week, when it attained a maximum sustained wind speed of 185mph (298km/h) as it moved over the Philippine Sea - the equivalent of a Category 5 hurricane.
While initially categorised as a "super typhoon", Champi weakened on Wednesday as it continued barrelling through the sea.
This is shaping up to be one of the most active typhoon seasons the region has seen in years. Champi is Japan's 30th typhoon of the year - making this the first time since 2013 that the country has seen so many storms.
The recent frequency of storms is due to an unusually strong El Niño - a naturally occurring phenomenon that hits the Pacific Ocean every few years and can trigger extreme weather events, from droughts to typhoons.
English (US) · 
Turkish (TR) · 