Ashleigh Barty fights back to beat Belinda Bencic at WTA Finals

Barty's 5-7 6-1 6-2 win in the Red Group also means the Australian is set to seal the year-end world number one ranking for the first time.

Bencic had started well in a tight first set, saving a break point before pouncing on two Barty double faults in the 11th game to wrest the advantage.

But a rising unforced error count would cost Bencic dear in the second set, not to mention ill fortune when a net cord enabled Barty to break for the first time for a 2-1 lead.

From that moment Bencic's resistance fell away and, with Barty growing in confidence behind her booming serve, the match descended into a one-sided affair.

Barty, whose year-end status as world number one will be confirmed when she steps on court for her second round-robin game, told reporters: "It certainly wasn't something that was in my realm at the start of the year.

"First and foremost we'd love to finish off the week here really strongly, then we can worry about celebrating that little thing."

Also in the Red Group, third seed Naomi Osaka won an entertaining opener against Petra Kvitova 7-6 (1) 4-6 6-4.

Osaka battled from a break down before easing through a first-set tie-break, and looked set to cruise to victory when she broke Kvitova for a commanding lead in the second.

But Kvitova stormed back to take the second set and force a decider, which Osaka would dominate to extend her current winning streak to a career-best 11 matches.

Aryna Sabalenka won the biggest title of her career to date as she defeated top seed Kiki Bertens to win the WTA Elite Trophy in Zhuhai.