The bellwether index lost 12.1 percent from a year earlier to end at 20,014.77 in roller-coaster trade, while the broader Topix index was down 17.8 percent in 2018 at 1,494.09.

On Friday alone, the Nikkei lost 0.31 percent and the Topix fell 0.50 percent as investors cashed in ahead of the New Year holidays.