Late entry Kiki Bertens has crashed Ashleigh Barty's party with an unlikely victory over the world No.1 at the WTA Finals in Shenzhen.

Barty officially clinched the year-end top ranking just by taking to the court on Tuesday night, but celebrations were soured with a 6-3 3-6-4 loss to the Dutch alternate.

Barty was to have played Naomi Osaka in her second round-robin match before the Japanese world No.3 withdrew from the season finale with a shoulder injury.

The French Open champion appeared to be cruising towards victory and a place in the semi-finals, only to collapse from a set and a service break up. Barty must now beat Petra Kvitova in her last group match on Thursday night to secure a spot in the final four and stay in the hunt for the record $US4.1 winner's cheque.

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.

But the 23-year-old will be disappointed not to have put Bertens away in straight sets.

She led 3-2 with a break in the second only to inexplicably drop eight successive games to suddenly trail 4-0 in the deciding set.

Barty fought back to 3-4, but Bertens held firm to close out the contest after two hours and eight minutes.

Bertens faces Belinda Bencic next on Thursday night in her bold quest to become the first alternate to make the semi-finals at the season-ending championships. Barty's Fed Cup teammate Samantha Stosur earlier won her second doubles match in Shenzhen with Zhang Shuai before paying tribute to her compatriot's otherwise 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 highlights in 2019 include winning her first Premier Mandatory title in Miami, reaching the Australian Open quarter-finals 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.