World number one Tennis player Serena Williams has opted to pull out of the WTA Finals and end her season early following a roller coaster campaign that has taken its toll on her body. She said she will not play again this year after also pulling out of next week's China Open.The American player today said that she taking the decision to focus on her health. The 21-times Grand Slam singles champion Williams, 34, has not played since her shock loss to Italy's Roberta Vinci in the semi-finals of the US Open last month, which ended her bid to claim all four grand slam titles in the same season.