Aussie coach's epic mid-match pep talk inspires comeback win

Simona Halep says Aussie coach Darren Cahill's rousing mid-match pep talk inspired her remarkable come-from-behind victory over Bianca Andreescu at the WTA Finals in Shenzhen.

Halep credited her Australian super-coach for dragging her through an emotional rollercoaster against the teenage sensation from Canada.

The Romanian rallied from a set and match point down to pull off a thrilling 3-6 7-6 (8-6) 6-3 victory over the US Open champion in their much-anticipated opening round-robin encounter.

Reunited with Cahill after his 12-month hiatus, Halep ushered her trusted mentor to the court for regular pep talks as she struggled to find a solution to Andreescu's cunning tactics and game style.

So reliant was Romania's former world No.1 on Cahill that the umpire even denied her one session for excessive mid-match coaching.

"I called him so many times because I missed him this year. I just wanted to make sure I have him the full charge, let's say," Halep said.

"I ask once more when she got the medical (time-out), and they said 'you had already too many'. I said 'okay'.

"He told me good advice. He gave me advice that I'm playing okay, even if I thought I'm not playing very solid during the match.

Cahill could be seen rallying a dejected-looking Halep with stirring words of encouragement, imploring the World No.5 to back herself more on second serve.

"He told me that I'm okay and I can serve better, which is true. Even if I didn't serve great in this match, in the end was a little bit better.

"Yeah, advice about the game, so he helped me.

"I'm happy that Darren is back. He gave me a little bit of power also."

As resilient as Halep was, Andreescu looked set to add yet another big-name scalp to her stunning 2019 CV before a back injury clearly hindered the 19-year-old in the deciding set.

Injury cloud hangs over Andreescu

The world No.4 twice required treatment after missing her match point with Halep serving at 5-6 in the second set and then refused to guarantee she'd be okay to play her remaining two group matches.

"We'll see," Andreescu said.

"I really don't want to talk about my back right now ... 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.

"Pretty disappointed."

Halep's recovery job came after Elina Svitolina opened her title defence with a gritty 7-6 (14-12) 6-4 win over Karolina Pliskova in the night's opening match from the Purple Group.

But it was Halep's first-time battle with Andreescu that captivated Chinese fans for more than two-and-a-half hours.

Andreescu, who this time last year was ranked No.185 in the world and about to play a $US60,000 ITF Circuit event in Toronto, promised to bring a "different vibe" to the court - and did so.

Mixing Nick Kyrgios-like tweeners with deft drop shots and forehand slices, Andreescu came within a point of improving her record over top-10 rivals to a crazy nine and one.

But after missing her opportunity, Andreescu - if she can carry on - now must beat both Svitolina and Pliskova to make the semi-finals on debut at the elite eight-woman season finale.

She meets Pliskova on Wednesday, when Halep plays Svitolina for a likely semi-final berth.

Red Group leaders Ashleigh Barty and Naomi Osaka, who both won their openers, clash in Tuesday night's first match before Petra Kvitova plays Belinda Bencic.