World number one Victoria Azarenka remains in suspense as to whether she will capture the year-end top spot after losing to Serena Williams for the fifth time in five meetings this year on Thursday.
The Belarusian's 6-4, 6-4 loss to the American in the WTA Championships means she may have to beat Li Na, the former French Open champion from China, on Friday to be sure of becoming number one at the end of 2012.
"I have been practising hard and expected to play better. Hopefully as time goes on, I will get better. If not, I hope to play better next year," said Williams, who also defeated Azarenka in the US Open final last month.
There was just a phase of about 20 minutes in the second set, as Azarenka surged to a 3-0 lead, when it seemed she might break her losing sequence against the Olympic, US Open and Wimbledon champion.
Otherwise she seemed too tense to do herself justice, as evidenced by nine double faults, which included two with disastrous timing at the end of each set.
Williams was mostly not at her best either, often being made to scramble by Azarenka's aggressive approach.
But she improved as the match went on, was severe on Azarenka's second serve, and was much the more effective on the bigger points.
The American world number three finished with 35 winners and 11 aces.
"When I did the right things I was in control much of the time, but I didn't have the consistency today," said Azarenka, who declined to make Wednesday night's tiring three-hour match with Angelique Kerber an excuse.
"I had too many opportunities I didn't use. I will be a little bit sad, but I need to bounce back straight away."
Williams had already qualified for the semi-finals with her second win on Wednesday, against Li Na, and now finishes her group matches unbeaten.
It means that if Li beats Azarenka, and if Maria Sharapova, who is already sure of a semi-final place, were to go on to regain the WTA Championships title, the Russian would snatch the year-end top spot.
Hence Azarenka's desire to refocus fast as she faces a must-win match against Li Na.
"I will give everything to win tomorrow," she said.
"I will give two hundred per cent. I have another opportunity to stay in the tournament and to keep the dream alive."
Earlier Li looked in intermittently good form, keeping alive hopes of reaching the semi-finals with a resilient performance in a straight sets success over Kerber.
The former French Open champion's 6-4, 6-3 win over the fifth-seeded German may well have been helped by the energy Kerber expended while reaching match points during her massive three-hour effort against Azarenka on Wednesday.
Kerber broke serve at once and held her lead until 4-2. But once Li had broken back for 4-4 she always looked the more likely winner, more nimble of step, more flat-hitting of ground stroke, and eventually more motivated for success too.
By the end of the first set Kerber was becoming wayward with her serve and was looking discouraged, and which prompted some very intense in-your-face exhortations from coach Torben Beltz.
Li by contrast was nodding at comments from her new coach Carlos Rodriguez, of whom she says: "He makes me a little bit stronger in the mind as well as in tennis."
When Kerber served a double fault at 4-5, advantage receiver, it changed the match. Li broke again at once in the second set, and although Kerber bravely got a break back, Li broke twice more, penetrating more and more with her raking drives.
Kerber's resistance never completely disappeared and included one innovative double-handed shovel shot which launched a remarkable lob over her head while facing the wrong way.
But it made little difference, and if the eight-seeded Li can do well against Azarenka on Friday, she might yet qualify against the odds.