Williams, has won her fifth U.S. Open championship and 17th Grand Slam title.
Williams defeated second seed Victoria Azarenka of Belarus 7-5, 6-7, 6-1 Sunday in New York in a rematch of last year's title match, also won by Williams.
With the victory, Williams became the oldest U.S. Open women's winner since tennis turned professional 45 years ago. Williams turns 32 later this month.
Sunday's match, played in windy conditions, lasted two hours and 45 minutes, becoming the longest U.S. Open women's final since timing began in 1980.
The men's final Monday features top seed Novak Djokovic of Serbia against number two Rafael Nadal of Spain.