Wimbledon champion Simona Halep survived match point before beating Canadian teenager Bianca Andreescu in a pulsating WTA Finals clash in Shenzhen.
The 28-year-old Romanian was on the brink at 6-5 in the second set before taking her third set point in the tie-break to force a decider.
US Open champion Andreescu required lengthy treatment on her back before Halep clinched a 3-6 7-6 (8-6) 6-3 win.
Earlier on Monday, defending champion Elina Svitolina beat Karolina Pliskova.
Andreescu plays Czech second seed Pliskova on Wednesday in their second round-robin match, with the 19-year-old saying she will spend the next 48 hours trying to overcome the back problem which hampered her against Halep.
"In the third set, I really don't want to blame my back, but I felt like I didn't have the same power as I did in the first and second set," Andreescu said.
Ukrainian world number eight Svitolina edged an 18-minute first set tie-break before sealing a 7-6 (14-12) 6-4 win.
Both players broke each other early in the opening set before the marathon tie-break which Svitolina clinched when the Czech's backhand landed wide.
Svitolina's break in the seventh game of the second set proved decisive as she claimed a seventh successive victory at the WTA Finals.
Svitolina, 25, 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."
The WTA Finals sees the world's top eight 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.