"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 season with a 58-4 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 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. She finishes behind No. 2 Sharapova and Victoria Azarenka, who ends the year as the top-ranked player in the world despite losing to Sharapova in the semifinals.
"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."
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.
She broke Sharapova's serve to start the second set and was never really threatened again. Williams hit a powerful return on her first match point and had 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. 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.
"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."
Williams also became the oldest player, at age 31, to win the year-end championships and has seven titles this year.
Shortly after winning Wimbledon two years ago, Williams cut her foot on glass at a restaurant, leading to a series of health problems, including being hospitalized for blood clots in her lungs.
She also injured her ankle at the start of the year in Brisbane, Australia.