(Updates after Williams reaches semi-finals) * Williams assured of year-end top ranking for fourth time * American qualifies for WTA Finals last four By John O'Brien SINGAPORE, Oct 24 (Reuters) - Serena Williams clinched the year-end world number one ranking for the fourth time without hitting a ball on Friday after Maria Sharapova, the only woman with any chance of overtaking her, was eliminated from the WTA Finals. ... More...