Japan's Naomi Osaka dismissed Jennifer Brady in straight sets to win the Australian Open and became only the third player after Monica Seles and Roger Federer to win their first four major finals.
Osaka edged a tight first set and broke twice in the second to win 6-4, 6-3 in 77 minutes in front of thousands of fans at Rod Laver Arena.
Williams will now face Maria Sharapova for the title after the Russian second seed beat compatriot Ekaterina Makarova 6-3 6-2 earlier on Rod Laver Arena.
The 2008 champion had to take a medical time-out between the second and third sets for treatment on an undisclosed injury. She was grimacing when bending over and grabbing her lower back during the final set.
Li Na wore down Agnieszka Radwanska with a barrage of power-hitting to beat the fourth seed 7-5 6-3 and advance to her third semi-final at the Australian Open on Tuesday.
Roger Federer, Rafa Nadal and other top tennis players hammed it up in exhibition matches on the eve of the Australian Open to raise money for victims of the country's devastating floods on Sunday.