The American crushed Petra Kvitova 6-2 6-3 to complete her stroll through the Red Group at the season-ending WTA Championships, looking unbeatable in the process.
World number one Williams, looking to retain her title, has won all three matches at the Sinan Erdem Dome without a dropped set, leaving her rivals to scrap over the runners-up spot in the section.
China's Li Na is also unbeaten in the White Group after a 6-3 2-6 6-3 victory over Serbia's Jelena Jankovic on Thursday.
The world number five was forced to fight hard by Jankovic in the only match in the tournament to go the distance so far.
With second seed Victoria Azarenka playing her final match on Friday, Li is still not guaranteed to reach the last four as Jankovic will be expected to beat already eliminated Sara Errani.
Li leads the standings but she, Azarenka and Jankovic could all end up with two wins apiece.
"Matches are always tough with Jelena because she is a very different player. Today I was running a lot, forward and backward on the court," Li said.
"Now it is one foot in the semis but another foot still out of the door so I have to see how it goes tomorrow."
Italy's Errani has lost both her matches in straight sets and cannot qualify because Jankovic snatched a set from Li.
The scenario in the Red Group in terms of who joins Williams in the last four is a simple winner-take-all battle between Ktivova and Germany's Angelique Kerber on Friday.
Kerber beat Poland's Agnieszka Radwanska 6-2 6-2 to claim her first victory at the event, having lost all three matches in 2012.
World number four Radwanska will return home without even a set to her name but was not too concerned.
"I think it was a good year," she said. "Still top five, I'm very happy about that especially as I really played consistent tennis pretty much the whole year.
"Of course I've had some tough losses but altogether I will say it's been a good year."
(Editing by Tony Jimenez)
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