Victoria Azarenka faces a wait to see if she can end 2012 as world number one after being beaten 6-4 6-4 by Serena Williams at the WTA Championships.
The American led the head to head between the duo 10-1 before Thursday's match and came back from 3-0 down in the second set to secure her 11th win.
Azarenka can still be caught in the end-of-year ranking race by current world number two Maria Sharapova.
Elsewhere China's Li Na beat German Angelique Kerber 6-4 6-3.
World number three Williams, who beat Azarenka in last month's US Open final, had already secured her place in the semi-finals after beating both Li and Kerber.
"I've been struggling with my serve in the first two rounds and I knew I could do much better, she said.
"I knew I needed to play better to win against Victoria, who has had a stellar year."
Azarenka must now beat Li on Friday to make sure of her place in the semi-finals of the season-ending event in Turkey.
"I have been practising hard and expected to play better," said the Belarusian.
"When I did the right things I was in control much of the time, but I didn't have the consistency today."
She added: "I will give everything to win tomorrow. I will give 200%. I have another opportunity to stay in the tournament and to keep the dream alive."
In the other group Sam Stosur, who was a replacement for Petra Kvitova who withdrew through illness, was beaten 2-6 6-3 0-6 by Italy's Sara Errani.