The 32-year-old world No.1 beat Li Na 2-6 6-3 6-0 in the final in Istanbul on Sunday, ending 2013 with $US12,385,572 ($A12.90 million) in prizemoney - obliterating the previous mark of $US7,923,920 ($A8.26 million) set by Victoria Azarenka last year.
It's also the third-highest single season prize money total in tennis history, men's and women's - only Novak Djokovic's 2011 and 2012 totals are higher.
Williams had admitted to feeling physically exhausted before, during and after a fraught semi-final with Jelena Jankovic on Saturday and - at 3-3 in the second set on Sunday - she looked in danger of a surprise defeat.
"But I had to keep going, I hung in there, and it's amazing to win this," said Williams.
"I've just had a really long year, and I'm just really excited, honestly.
"I can't believe I won. I was so tired. Honestly, did I really win? Because she played so well."
It's been a year to remember for Williams, whose victory made her the fourth player ever to win four or more titles at the event and the oldest champion.
She is also the first player to win 11 WTA titles in a season since 1997 when Martina Hingis won 12.
The triumph may persuade Williams that adding to her 17 grand slam titles during 2014 is certainly possible, and that winning another five, to overhaul Steffi Graf, might eventually come within reach too.
"I'm just overjoyed, to be honest," Williams said.
"It's really awesome. It's such a special moment. To finish the year No.1 in the world and win this title after 40 years of the WTA, it means even more."
Williams looked subdued and shackled after Li, the first Chinese player to make the final of the end-of-season showpiece and about to become the highest-ranked Asian woman at No.3 in the world, broke her serve twice in the first set.
Williams held on to her serve in the seventh game but the set was effectively gone.
The second set started with a mighty battle, which resulted in Williams hanging on to her serve after almost 12 minutes of effort and eight deuces. It was a big moment.
Next game she broke Li's serve and the mood of the match began to shift.
Although Li got back to 3-3 with some impressively positive driving, a double fault contributed to a lost service game which put her at 3-5.
By now Williams' survival instincts were engaged intently.
She closed out the set after another service game of several deuces, and when Li delivered her eighth double fault to lose the opening game of the final set the tide completely turned.
Li double faulted again to go 0-3 down and an encouraged Williams was hitting the ball better.