Top seed Victoria Azarenka saved two match points in the process of recording a 6-7 (11-13) 7-6 (7-2) 6-4 win over Angelique Kerber at the WTA Tour Championships in Istanbul.
The world No. 1 was in difficulty several times during a tough battle which lasted over three hours but eventually came through to begin her campaign at the season-ending event with an important win.
She is now second behind Serena Williams, who has played a match more, in the Red Group while Kerber is on the brink of elimination having lost twice.
Kerber signalled her intentions early on as she broke her opponent in the third game, but some looseness on her own serve allowed Azarenka back in and the rest of the set went with serve.
It took a marathon tie-break to finally separate the two, Kerber saving a total of five set points as she came back from 6-2 down to take it 13-11.
Kerber's momentum was checked as she dropped serve in the first game of the second set, only for an error-strewn eighth game allowing the German to break back to love.
Azarenka then went two match points down in her next service game but dug herself out of trouble to set the clash on course for another tie-break.
The Belorusian again took control from the start but this time was able to close things out to force a deciding set.
The pair traded breaks at the start of the last before Azarenka struck the decisive blow in the eighth, allowing her to serve out the win with ease.
"I was fighting so hard - at the beginning of the tournament it's quite hard to have a match like that," Azarenka told the WTA's official website.
"But all the best players are here and you have to show your best tennis. I don't think I did that, but at least I showed my best fight. I'm glad I lived this experience and overcame the challenges today."