The sisters last contested the Wimbledon 2009 final.
Williams is tied with Steffi Graf on 22 Grand Slam singles titles.
Williams' 22nd major title earned with a 7-5, 6-3 win over Germany's Angelique Kerber.
This is an unusual way to prepare for an important match.
The American is one major behind the retired star.
Rising British star's run ends at the semi-final stage in Melbourne as Serena Williams powers on.