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.

The Ukrainian world number eight 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.

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

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.

Elina Svitolina recorded her fourth consecutive victory over Karolina Pliskova