Serena Williams clinched her third win at the WTA Championships on Thursday with a 6-4 6-4 victory over Victoria Azarenka that prevented the Belarusian guaranteeing the end-of-year No.1 ranking.
World No.3 Williams, who had defeated Azarenka in the US Open final last month, had already made sure of her place in the semi-finals after beating Li Na of China and Angelique Kerber.
On Thursday, she hit 35 winners and 11 aces.
Azarenka must beat Li on Friday to make sure of her place in the semi-finals of the season-ending event.
Earlier on Thursday, Li kept alive her hopes of reaching the semi-finals with a resilient and polished performance in a straight sets win over Kerber.
The 6-4 6-3 win over the hard-working German might have also had something to do with the energy Kerber expended while earning match points in a massive three-hour effort against Azarenka the night before.
Kerber nevertheless started well, breaking serve almost at once and holding her lead until 4-2.
But once Li had broken back for 4-4, she was more nimble, more flat-hitting and, eventually, more motivated for success too.
By the end of the first set, Kerber was becoming wayward with her serve and looking discouraged, which prompted some very intense in-your-face exhortations from coach Torben Beltz.
Li by contrast was relaxed, 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.”
Li broke at once in the second set and, although Kerber bravely chiselled out a break back, the Chinese star broke twice more, penetrating increasingly with her raking drives.
If the eight-seeded Li can do well against Azarenka on Friday, she could yet qualify against the odds.