World number one Ashleigh Barty suffered a surprise three-set loss to last-minute addition Kiki Bertens at the WTA Finals on Tuesday.

Bertens was called in to take Naomi Osaka's spot after the Japanese suffered a right shoulder injury and the Dutchwoman promptly made an impact in the Red Group.

After losing the first set, Bertens battled back to triumph 3-6 6-3 6-4 in Shenzhen, claiming her first win over Barty at the sixth attempt.

Barty, who came from a set down to win her opening match against Belinda Bencic, was the first player to qualify for the season-ending tournament back in early September but she failed to justify her tag as favourite for the title in a largely disjointed display.

It was a contest in which breaks of serve were frequent and a run of five defined the opener, Barty edging that back-and-forth to nudge ahead.

She was a break up in the second as a similar pattern emerged, but a first-serve percentage of just 41 was always likely to prove problematic and Bertens was producing more winners to further tip the balance in her favour.

With Barty serving at 4-3 down, Bertens sensed her opportunity and was the primary aggressor, striking for the lines and putting the French Open champion on the back foot.

Having fought to take the contest into a third set, Bertens was in no mood to relent and she stormed into a 4-0 lead, even fluffing two break-point openings to make it 5-0.

Despite a rally that saw the gap cut to 4-3, the die was cast and Barty was consigned to defeat in a clash that lasted over two hours.