'A tough one': Ash Barty suffers epic WTA Finals collapse

Australian Open party pooper Petra Kvitova threatens to spoil Ashleigh Barty's end-of-year celebrations as well.

Barty officially clinched the year-end world No.1 ranking but squandered a chance to seal a semi-final spot at the WTA Finals after falling in three sets to Kiki Bertens.

Bertens, an emergency alternate, made the most of her late call-up as a replacement for the injured Naomi Osaka to upset Barty 3-6 6-3 6-4 in Shenzhen.

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Barty appeared to be cruising towards victory and a place in the final four, only to collapse from a set up and 3-2 with a service break after inexplicably losing eight successive games.

"When you give a player of Kiki's calibre a chance to get back into the match not once but twice, she's going to make you pay," the French Open champion said.

"Obviously a tough one."

A winner over Belinda Bencic in her tournament opener on Sunday, Barty was to face Osaka in her second round-robin encounter before the Japanese world No.3 withdrew with a shoulder problem only three hours before.

But the top seed refused to blame the late change for her loss.

Despite the defeat, Barty is the first Australian since Lleyton Hewitt in 2001 and 2002 to finish a season as world No.1.

No other Australian woman has ever managed the feat, not that Barty was in any mood to celebrate.

"I don't think the world No.1 ranking comes into it at all. It's a bonus, but it's not something that I've really been thinking about," she said.

"It's still a few days away yet, and it's certainly not something that I'm worried about.

"Overall, we know we're still alive in the tournament. That's the beauty of round-robin. It's a different format."

Barty must now beat Kvitova in her last group match on Thursday night to secure a spot in the semi-finals and stay in the hunt for the record $US4.1 million ($A5.9 million) winner's cheque.

Barty succumbed to Kvitova in this year's Australian Open quarter-finals before finally toppling the Czech in Miami for the first time in five meetings.

Barty also defeated the dual Wimbledon only three weeks ago in Beijing.

"I've played Petra a number of times this year. Each match has been exceptionally tough. We've kind of split them a little bit," Barty said.

"Conditions are different here. Changes things a little bit.

"We'll sit down tomorrow and figure out a way we'll nullify her strengths and get it back to my patterns as much as we can."

Barty's Fed Cup teammate Samantha Stosur, playing doubles in Shenzhen, said nothing could detract from Barty's spectacular season.

"It's just incredible. The year she's had anyone would dream of having once in their career," Stosur told AAP.

Apart from her grand slam breakthrough in Paris, Barty's other 2019 highlights include winning her first Premier Mandatory title in Miami, making the last eight in Melbourne and being the only woman to reach the second week at all four majors.

Barty also won titles on hard, clay and grass courts and has so far recorded a tour-best nine wins over top-10 rivals.

But she'll likely need at least one more to reach the semi-finals in Shenzhen.