Defending champion Elina Svitolina got the better of world number two Karolina Pliskova in an 18-minute first set tie-break before sealing a 7-6 (14-12) 6-4 win at the WTA Finals in Shenzhen.

Both players broke each other early in the opening set before the marathon tie-break which the Ukrainian clinched when the Czech's backhand landed wide.

Svitolina's break in the seventh game of the second set proved decisive.

It was the world number eight's seventh successive win in the WTA Finals.

Svitolina said: "That was probably one of the longest tie-breaks I have played.

"I was just trying to be ready for her big serves today. I tried to move quickly and react quickly. The slightly slower surface gave me a bit more time.

"It's a big privilege to return to this event. I still can't believe I won it last year."

US Open champion and world number four Bianca Andreescu of Canada takes on Romania's world number five Simona Halep in Monday's late match.

The WTA Finals event sees the world's top eight players compete in two round-robin groups of four, with the winners and runners-up advancing to the semi-finals.

Svitolina is in the Purple Group alongside Pliskova, Halep and Andreescu. The Red Group features Naomi Osaka, Ashleigh Barty, Petra Kvitova and Belinda Bencic.