Williams, who also finished No. 1 in 2002 and 2009, has won nine titles this season, including the French Open and U.S. Open. She has a record of 67-4 this year.
The 32-year-old Williams returned to the No. 1 spot in February, when she became the oldest player to top the rankings.
She is the seventh player to finish at least three seasons at No. 1, along with de Steffi Graf (eight times), Martina Navratilova (seven), Chris Evert (five), Lindsay Davenport (four), Justine Henin (three) and Martina Hingis (three).
Williams was interviewed on ABC's "Good Morning America" after her 2013 U.S. Open win. Watch a clip of the interview below (mobile users: http://bit.ly/1bcvzM6):