With a crowd of 13,000 looking on, the 29-year-old beat Williams 6-3, 6-3.
"I had no regrets at stopping my career on August 29 at the US Open, and I still don't have any. Sorry to stop you," said Clijsters.
"There's not just tennis in my life. I like cooking, gardening, reading, decorating the house, putting up the Christmas tree and running."
Clijsters retired for the first time in 2007 to give birth to daughter Jada with no plans to return, feeling she had accomplished her Grand Slam dream with a 2005 US Open title.
But she returned in 2009 and with only two tune-up events captured the US Open title - becoming the first mother to win a Grand Slam title since 1980, then defended the crown in 2010 and added a 2011 Australian Open title just for good measure.