Ash Barty has joined tennis legends Steffi Graf, Chris Evert and Serena Williams in securing the women's year-end world number one ranking for at least three straight seasons

Despite formally announcing she won't defend her season-ending crown at the rescheduled and relocated WTA Finals in Mexico next month, officials have confirmed Barty can't be surpassed in the 2021 rankings.

Aryna Sabalenka's quarter-final loss to Ekaterina Alexandrova at Moscow's Kremlin Cup sealed the deal for Barty, who holds a massive rankings lead over her rivals after a stellar five-title, six-final season.

The 2019 French Open champion added a second grand slam crown to her collection at Wimbledon in July as well as landing titles in Cincinnati, Stuttgart, Miami and Melbourne.