"Now that I can be honest, I really wanted to win," Williams said. "I wanted it so bad but I didn't want to say it ... It was really important for me to end the year with this title in particular."
Williams ended the year with a 59-4 record. Since her first-round loss at the French Open, the American is 31-1, winning Wimbledon, the Olympic gold medal and the U.S. Open.
But she will finish the year ranked No. 3, behind Sharapova, because she did not pay as well in the first half of the year.
"I had such a good year, it was important to end on a good note. it was good for my sanity to win," Williams said. "I really wanted it although I didn't need it."
Victoria Azarenka, beaten by Sharapova in the semifinals, ended the year as the top-player in the world.
In 2001 and '09 Williams also won the elite, year-end tournament that brings together the top eight players in the world.
Williams raced forward to reach a drop shot and put away a passing-shot winner but the seventh game still went to Sharapova after five deuces. But the American was pumping her fist and there was no holding her back as Williams closed out the set with an ace - one of 11 she had in the match.
"Today she had another great serving day against me," said Sharapova, who never had a break point.
Williams broke Sharapova's serve to start the second and was never really threatened again.
Williams hit a powerful return on her first match point. She finished with 40 winners, compared to Sharapova's 13.
Williams finished the tournament without dropping a set and she also beat Azarenka in round-robin play, one of her four wins against the No. 1 this year.
"If I'm playing well and doing everything right, It's pretty difficult to beat me, without trying to sound too full of myself," Williams said. "And I hate to lose."
The American has won 12 straight against opponents ranked No. 1 or No. 2 and has not lost to a player ranked in the top 2 since 2007.
Williams also became the oldest player, at age 31, to win the year-end championships and has seven titles this year.
But because she did not play as well at the start of the year following injuries and illness, Williams will have to settle for the No. 3 ranking despite dominating the tour in the past few months.
Shortly after winning Wimbledon two years ago, Williams cut her feet on glass at a restaurant, leading to a series of health problems, including being hospitalized for clots in her lungs.
She also injured her ankle at the start of the year in Brisbane.
Williams last finished the year as No. 1 in 2009. She has held the No. 1 ranking for a total of 123 weeks.