Victoria Azarenka's expected crowning as year-end No.1 will have to wait another night, after the Belarusian was denied 6-4, 6-4 by her nemesis Serena Williams, and left to play eighth seed Li Na for a place in the semi-finals of the $4.9 million WTA Championships.
What was effectively a dead rubber for Williams, who had already qualified for the last four with wins over Li and Angelique Kerber, was nevertheless an opportunity for the American to extend her career domination of Azarenka to 11-1. And so she did, in a 95-minute rematch of the US Open final Azarenka lost after serving for the title at 5-3 in the third set.
This latest contest also looked headed for a decider when Azarenka claimed a double-break to lead 3-0 in the second set. But Williams captured six of the last seven games, with 11 aces among her 35 winners as the Belarusian faltered on her second serve. The match, like the first set, finished with a double fault.
"I had too many opportunities that I didn't use," said a red-eyed Azarenka, who converted only three of her nine break points and now must return for what is a "win or go home" elimination clash with Li.
"She played really well. As usual she's on a roll and everything, but I feel like I didn't keep up the discipline I had, the right play I had. When I had the right play I was in control most of the times, but I just couldn't do it consistently today.
"I can still see the positive. I have tomorrow another opportunity to play and to win and I'm still in the tournament. I'll be sad for a little bit, but I'll need to bounce back strong for tomorrow."
Williams, though, continues to have her measure, and Azarenka is No.1 in ranking only when she plays the 15-time grand slam champion. "Maybe I'm doing the right things at the right time," said Williams, who has won 14 of her 15 sets against the Australian Open champion this year.
"I don't know what the edge is. I think every time you step out on that court it's a brand new time and there really is no edge."
Earlier, Li had ended Angelique Kerber's hopes of advancing by the end of their first set, the German's debut championships finishing at an unflattering 0-3, but less than 24 hours after her match-of-the-tournament against Azarenka, and with plenty to celebrate from a breakthrough season in which the 24-year-old won her first WTA title, reached the semis at Wimbledon and finished at No.5.