Despite formally announcing on Saturday 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.

In 2021, Barty is the only woman to have hoisted trophies on grass, clay and hard courts.

"No doubt the player of the year," esteemed coach and former Australian Fed Cup captain David Taylor told AAP.

"You look at Roger (Federer), he can tactically win with such an array of tactics and having coached on the WTA Tour for so long, you're often left with quite a very narrow set of tactics for most of your players on there.

"With Ash, she's dangerous on all surfaces, which she's obviously already proven, and she's won the two biggest grand slams on the most contrasting surfaces so she's got a lot of ways to win matches.

"The WTA has a lot of great ball strikers but she's a great tennis player and that's the big difference."

"I'm stating the obvious but she really is the best tennis player, not just the best ball striker."

Barty hasn't played since the US Open early last month and says her full focus now is on preparing for the Australian summer and another tilt at her home grand slam in Melbourne in January.

Ending her season prematurely will come at a price financially.

The Queenslander pocketed the biggest cheque in tennis history, for some $6.4 million, after winning the WTA Finals in Shenzhen in 2019 before last year's edition was cancelled because of the pandemic.

But not wanting to endure a second two-week stint in quarantine in barely two months, having returned home only last month, Barty has opted against travelling again this year.

"I wanted to let everyone know that I won't be competing in any further tournaments in 2021, including the WTA Finals in Mexico," Barty said on Saturday.

"It was a difficult decision but I need to prioritise my body and my recovery from our 2021 season and focus on having the strongest pre-season for the Australian summer.

"With ongoing challenges of travelling back to Queensland and quarantine requirements, I am not willing to compromise my preparation for January.

"I wish the WTA team and the players all the best for a successful WTA Finals and the rest of the year."

Australian Associated Press