The world number one went into the end-of-season tournament as the hot favourite to retain her title and three comfortable round-robin victories have only enhanced her status.
This time her opponent was fifth seed Petra Kvitova, who won the title at the Sinan Erdem Dome in 2011 and impressed in beating Agnieszka Radwanska in her first match.
But the Czech did not live up to her billing as Williams eased to a 6-2 6-3 victory to guarantee top spot in the red group.
Had Kvitova taken any of the three break points she created in the first game, it might have been a different story, but Williams saved them all.
The American then broke serve immediately and again in the eighth game to clinch the opening set.
Williams' all-round level was simply a cut above, as it has been in virtually every match this season.
Kvitova made too many errors to put any real pressure on, and when it looked like she might one of Williams' 11 aces usually helped the defending champion out.
Williams did not lose a match in both her most recent appearances at this tournament in 2009 and 2012 and Thursday's night's win took her run of successive victories to 13, second only to Martina Navratilova's 21.
Kvitova, who has now failed to beat Williams in all five of their meetings, will still qualify for the semi-finals if she beats Germany's Angelique Kerber on Friday.
Kerber posted her first win at the WTA Championships earlier with a 6-2 6-2 thrashing of Radwanska, whose record after a miserable week read six sets played, six sets lost.
In the white group, Li Na has one foot in the semi-finals but she knows there is still work to be done.
The fourth seed prevailed 6-3 2-6 6-3 over Jelena Jankovic in the first three-set match of the tournament.
It was Li's second victory from two round-robin games after she also beat Sara Errani, and the Chinese player is now in pole position to qualify from her section.
A set against Victoria Azarenka on Friday would put her through but a straight-sets loss would leave her extremely vulnerable.
It is the first time in the three years Li has qualified for the year-end championships that she has managed two wins, and she said: "Of course it was tight but at least it is better than the last two years.
"Now I have one foot in the semis but another foot is still out of the door. If I lose easy tomorrow, I already maybe have no chance. So I still have to fight tomorrow."
It was a match of many momentum swings, but Jankovic was always playing catch-up, and after two hours and 11 minutes it was Li who came through.
Li had lost only three games to Jankovic in their most recent clash at the US Open, and it looked like this match might go the same way when the fourth seed won the opening three games.
Jankovic, an impressive winner over Azarenka on Wednesday, fought back to level only for Li to reel off another run of three games to take the opening set.
There was a similar pattern in the second set, but this time it was Jankovic who was the dominant player before they switched again in the deciding set.
Jankovic is still in a good position to reach the last four, though, knowing victory over the already-eliminated Errani in the final match on Friday would definitely be enough.
The Serbian said: "Overall Li played a great match. She was hitting the ball so well, and I was up there. I was very close. It was just a few points at the end that made a difference.
"So it's of course disappointing but I still have another match to go tomorrow and hopefully I can do it and give myself a chance to qualify for the semis."