Updated: Osaka beats Kvitova, Barty blasts Bencic in thrilling WTA Finals opener

Naomi Osaka and Ashleigh Barty both won three-set matches at the WTA Finals Sunday in a gruelling opening day of the $14 million round-robin tournament.

Barty joined the Japanese sensation at the top of Red Group with a 5-7, 6-1, 6-2 victory over Belinda Bencic in one hour and 56 minutes. The result secured the year-end No.1 ranking for the Australian.

"I gave Belinda too many looks at second serves in the first set," Barty told reporters. "That was the key for the second and third sets ... try to dominate a little bit more with the first serve."

In-form Osaka outlasted Petra Kvitova 7-6 (7/1), 4-6, 6-4 in two hours and 39 minutes to notch her first victory at the WTA Finals after a winless campaign in her debut appearance last year. She stretched her current winning streak to 11 matches having claimed titles in Osaka and Beijing.

In a rematch of this year's tense Australian Open final, world No.6 Kvitova came out with intent until she stuttered in the eighth game of the opening set with three double faults to squander the game.

Osaka held off two break points in the ninth game before dominating the tiebreak to draw first blood.

It seemed to shake the Czech's confidence but...