Melbourne - Australian Open champion Naomi Osaka is the youngest woman to make her debut at No 1 in nine years and the first man or woman from Asia to lead the tennis rankings.
Simona Halep's 48-week stay atop the WTA ended on Monday. She slid to No 3 after a fourth-round loss to Serena Williams.
Williams missed the tournament last year after having a baby; her run to the quar-terfinals this time pushed her up five places to No 11.
The 21-year-old Osaka's second consecutive major title helped her rise three spots, while Australian Open runner-up Petra Kvitova went from No 6 to No 2.
Men's champion Novak Djokovic remained No 1 in the ATP rankings. Runner-up Rafael Nadal stayed at No 2, but Roger Federer's fourth-round exit dropped him from No 3 to No 6.
South Africa's Kevin Anderson is at No 5 ahead of Federer.