ISTANBUL - Serena Williams’ motivating factor in her WTA Championships triumph on Sunday was to save her own sanity, the American said after beating Maria Sharapova in the final of the season-ending tournament.Williams capped off a memorable 2012 on the circuit with a 6-4 6-3 victory as she claimed her seventh WTA title of year.In a near faultless performance by the 15-times grand slam tournament champion, Williams never faced a break point during the final and was able to produce winners every time she was put under pressure by her Russian opponent.The 31-year-old, who also lifted singles silverware this year at Wimbledon, the U.S. Open, the London Olympics, Madrid, Charleston and Stanford, said it was important to end the season in style if only to preserve her mental state."I wanted it so bad, but I didn’t want to say it,” Williams told reporters. "For my own sanity I really wanted it, even though I didn’t need it. I really wanted to end on a good note.”After starting the year slowly, partly due to an ankle injury sustained in Brisbane during the first week of the year, Williams has been a supreme force ending 2012 with a 58-4 win/loss record.